Chapter XXI: The Fishes of the Deep Sea (10)

_Body covered with scales; a series of keeled scales along each side of the belly. Margin of the upper jaw formed by the intermaxillaries mesially, and by the maxillaries laterally. Lower pharyngeals united into a single bone. Dorsal fin opposite the anal, belonging to the caudal portion of the vertebral column. Adipose fin none. Air-bladder generally present, simple, sometimes cellular, without pneumatic duct. Pseudobranchiæ hidden, glandular. Stomach not distinct from the intestine, which is quite straight, without appendages._

The fishes of this family are chiefly marine, some living in the open ocean, whilst others have become acclimatised in fresh water; many of the latter are viviparous, all the marine forms being oviparous. They are found in all the temperate and tropical zones. Carnivorous.

This family is represented in the strata of Monte Bolca by rare remains of a fish named _Holosteus_, allied to _Belone_ or _Scombresox_, and by a species of _Belone_ in the miocene of Licata.

BELONE.--Both jaws are prolonged into a long slender beak. All
the dorsal and anal rays connected by membrane.

The long upper jaw of the “Gar-pike” is formed by the intermaxillaries, which are united by a longitudinal suture. Both jaws are beset with asperities, and with a series of longer, conical-pointed, widely-set teeth. Skimming along the surface of the water, the Gar-pike seize with these long jaws small fish as a bird would seize them with its beak; but their gullet is narrow, so that they can swallow small fish only. They swim with an undulating motion of the body; although they are in constant activity, their progress through the water is much slower than that of the Mackerels, the shoals of which sometimes appear simultaneously with them on our coasts. Young specimens are frequently met in the open ocean; when very young their jaws are not prolonged, and during growth the lower jaw is much in advance of the upper, so that these young fishes resemble a _Hemirhamphus_. About fifty species are known from tropical and temperate seas, _Belone belone_ being a common fish on the British coast. Its bones, like those of all its congeners, are green; and therefore the fish, although good eating, is disliked by many persons. Some species attain a length of five feet.

SCOMBRESOX.--Both jaws are prolonged into a long slender beak. A
number of detached finlets behind the dorsal and anal fins.

The “Saury” or “Skipper” resemble the Gar-pike, but the teeth in the jaws are minute; they seem to feed chiefly on soft pelagic animals. In their habits they are still more pelagic; and the young, in which the beak is still undeveloped, are met with everywhere in the open ocean, in the Atlantic as well as in the Pacific. The European species, _Sc. saurus_, is not rare on the British coast; four other species have been described, closely allied to _Sc. saurus_.

HEMIRHAMPHUS.--The lower jaw only is prolonged into a long
slender beak.

In the young both jaws are short; the upper is never prolonged, the intermaxillaries forming a triangular, more or less convex, plate. The “Half-beaks” are common between and near the tropics; some forty species are known, none of which attain to the same length as the Gar-pike, scarcely ever exceeding a length of two feet. Some of the tropical species live in fresh water only; they are of small size and viviparous.

ARRHAMPHUS.--Mouth formed as in _Hemirhamphus_, except that the
lower jaw is not produced into a beak. Pectoral fins of moderate
length.

One species (_A. sclerolepis_) from the coast of Queensland (not New Zealand); it may be regarded as a _Hemirhamphus_, with retarded development of the lower jaw.

EXOCOETUS.--Jaws short, intermaxillaries and maxillaries
separate. Teeth minute, rudimental, and sometimes absent. Body
moderately oblong, covered with rather large scales. Pectorals
very long, an organ of flying.

Forty-four different kinds of “Flying-fishes” are known from tropical and sub-tropical seas; some have a very wide range, whilst others seem to remain within one particular part of the ocean; thus, the species figured, _E. callopterus_, has been hitherto found on the Pacific side of the isthmus of Panama only. Their usual length is about 10 or 12 inches, but specimens of 18 inches have been caught. They always live in shoals, and their numbers at certain times and localities are immense; thus, at Barbadoes many boats engage in their capture, as they are excellent eating. The pectorals are in the various species of unequal length; in some they extend to the anal fin only; in others (and these are the best fliers) to the caudal. A few have curious, barbel-like appendages at the lower jaw, which may disappear with age or be persistent throughout life. The literature on the subject of Flying-fishes is very extensive, and great diversity of opinion exists among observers as regards the mode and power of their flight; but the most reliable agree that the fishes do not leave the water for the purpose of catching insects (!), and that they are unable to move their fins in the manner of a bat or bird, or to change voluntarily the direction of their flight, or to fly beyond a very limited distance. The most recent enquiries are those of K. Möbius (“Die Bewegungen der Fliegenden Fische durch die Luft,” Leip. 1878, 8vo), the chief results of which may be summed up thus: Flying-fish are more frequently observed in rough weather and in a disturbed sea than during calm; they dart out of the water when pursued by their enemies, or frightened by an approaching vessel, but frequently also without any apparent cause, as is also observed in many other fishes; and they rise without regard to the direction of the wind or waves. The fins are kept quietly distended, without any motion, except an occasional vibration caused by the air whenever the surface of the wing is parallel with the current of the wind. Their flight is rapid, but gradually decreasing in velocity, greatly exceeding that of a ship going 10 miles an hour, and a distance of 500 feet. Generally, it is longer when the fishes fly against than with or at an angle to the wind. Any vertical or horizontal deviation from a straight line is not caused at the will of the fish, but by currents of the air; thus they retain a horizontally straight course when flying with or against the wind, but are carried towards the right or left whenever the direction of the wind is at an angle with that of their flight. However, it sometimes happens that the fish during its flight immerses its caudal fin in the water, and by a stroke of its tail turns towards the right or left. In a calm the line of their flight is always also vertically straight, or rather parabolic, like the course of a projectile, but it may become undulated in a rough sea, when they are flying against the course of the waves; they then frequently overtop each wave, being carried over it by the pressure of the disturbed air. Flying-fishes often fall on board of vessels, but this never happens during a calm, or from the lee side, but during a breeze only, and from the weather side. In daytime they avoid a ship, flying away from it; but during the night, when they are unable to see, they frequently fly against the weather-board, where they are caught by the current of air, and carried upwards to a height of 20 feet above the surface of the water, while, under ordinary circumstances, they keep close to it. All these observations point clearly to the fact that any deflection from a straight course is due to external circumstances, and not to voluntary action on the part of the fish.

TENTH FAMILY--ESOCIDÆ.

_Body covered with scales; barbels none. Margin of the upper jaw formed by the intermaxillaries mesially, and by the maxillaries laterally. Adipose fin none; the dorsal fin belongs to the caudal portion of the vertebral column. Stomach without blind sac; pyloric appendages none. Pseudobranchiæ glandular, hidden; air-bladder simple; gill-opening very wide._

This family includes one genus only, _Esox_, the “Pikes,” inhabitants of the fresh waters of the temperate parts of Europe, Asia, and America. The European species, _E. lucius_, inhabits all three continents, but the North American waters harbour five, or perhaps more, other species, of which the “Muskellunge,” or “Maskinonge” (_E. estor_) of the Great Lakes attains to the same large size as the common Pike. The other species are generally called “Pickerell” in the United States.

Fossil Pike, belonging to the existing genus, have been found in the freshwater-chalk of Oeningen, and in the diluvial marl of Silesia. Remains of the common Pike occur in abundance in quaternary deposits.

ELEVENTH FAMILY--GALAXIIDÆ.

_Body naked, barbels none. Margin of the upper jaw chiefly formed by the intermaxillaries, which are short, and continued by a thick lip, behind which are the maxillaries. Belly rounded; adipose fin none; dorsal opposite to anal. Pyloric appendages in small number. Air-bladder large, simple; pseudobranchiæ none. The ova fall into the cavity of the abdomen before exclusion._

Small freshwater fishes of the southern hemisphere, belonging to two genera, _Galaxias_ and _Neochanna_. Of the former genus five species are found in New Zealand, where this type is most developed, three in New South Wales, two in Tasmania, and four in the southern extremity of South America. Their native name in New Zealand is “Kokopu,” and they were dignified with the name of “Trout” by the settlers before the introduction of true Salmonidæ. They rarely exceed a length of eight inches. _Neochanna_ is a degraded form of _Galaxias_, from which it differs by the absence of ventral fins. This fish has hitherto been found only in burrows, which it excavates in clay or consolidated mud, at a distance from water.

TWELFTH FAMILY--MORMYRIDÆ.

_Body and tail scaly; head scaleless; barbels none. The margin of the upper jaw is formed in the middle by the intermaxillaries, which coalesce into a single bone, and laterally by the maxillaries. Sub- and inter-operculum present, the latter very small. On each side of the single parietal bone a cavity leading into the interior of the skull, and covered with a thin bony lamella. All the fins are well developed, in_ Mormyrus; _or caudal, anal, and ventral fins are absent, in_ Gymnarchus. _No adipose fin. Pseudobranchiæ none; gill-openings reduced to a short slit. Air-bladder simple. Two coeca pylorica behind the stomach._

This family is characteristic of the freshwater fauna of tropical Africa. Of _Mormyrus_ (including _Hyperopisus_ and _Mormyrops_), fifty-one species are known, of which eleven occur in the Nile. Some attain a length of three or four feet, others remain small. Their flesh is said to have an excellent flavour. The species figured (and probably other allied species) was an object of veneration to the ancient Egyptians, and, therefore, frequently occurs in their emblematic inscriptions. They abstained from eating it because it was one of three different kinds of fishes accused of having devoured a member of the body of Osiris, which, therefore, Isis was unable to recover when she collected the rest of the scattered members of her husband.

The _Mormyri_ possess a singular organ on each side of the tail, without electric functions, but evidently representing a transitional condition from muscular substance to an electric organ. It is an oblong capsule divided into numerous compartments by vertical transverse septa, and containing a gelatinous substance. The _Mormyri_ differ much with regard to the extent of the dorsal and anal fins, the former sometimes occupying the greater portion of the length of the back, sometimes being much shorter and limited to the tail. In some the snout is short and obtuse, in others long and decurved, with or without appendage.

Of _Gymnarchus_ one species only is known, _G. niloticus_, which occurs in the Nile and West African rivers, and attains a length of six feet. The form of its body is eel-like, and each jaw is armed with a series of incisor-like teeth. Like _Mormyrus_, _Gymnarchus_ possesses a pseudo-electric organ, thickest on the tail, tapering in front, and extending nearly to the head. It consists of four membranaceous tubes intimately connected with the surrounding muscles, and containing prismatic bodies arranged in the manner of a paternoster. The air-bladder of Gymnarchus is cellular, very extensible, and communicates with the dorsal side of the œsophagus by a duct possessing a sphincter.

[See _Erdl_, Münchner Gelehrte Anzeigen, 1846, xxiii., and
_Hyrtl_, Denkschr. Akad. Wiss. Wien. 1856. xii.]

THIRTEENTH FAMILY--STERNOPTYCHIDÆ.

_Body naked, or with very thin deciduous scales; barbels none. Margin of the upper jaw formed by the maxillary and intermaxillary, both of which are toothed; opercular apparatus not completely developed. Gill-opening very wide; pseudobranchiæ present or absent; air-bladder simple, if present. Adipose fin present, but generally rudimentary. Series of phosphorescent bodies along the lower parts. The eggs are enclosed in the sacs of the ovarium, and excluded by oviducts._

Pelagic and Deep-sea fishes of small size.

STERNOPTYX.--Trunk much elevated and compressed, with the trunk
of the tail very short. Body covered with a silvery pigment,
without regular scales; series of phosphorescent spots run along
the lower side of the head, body, and tail. Cleft of the mouth
wide, vertical, with the lower jaw prominent. Jaws armed with
small teeth. Eyes rather large, and although lateral, directed
upwards and placed close together. Ventral fins very small. A
series of imbricate scutes runs along the abdomen, forming a
kind of serrature. The dorsal fin is short, and occupies about
the middle of the length of the fish; it is preceded by the
first commencement of the formation of a spinous dorsal, several
neural spines being prolonged beyond the dorsal muscle forming a
triangular osseous plate. Adipose fin rudimentary; anal short;
caudal forked.

These small fishes are now and then picked up in the Mediterranean and Atlantic. According to the dredging-records of the “Challenger,” they and the allied genera _Argyropelecus_ and _Polyipnus_ would descend to depths of respectively 1100 and 2500 fathoms; but the form of their body and their whole organisation render this statement very improbable; they most likely live at a small depth during the daytime, coming to the surface at night, like many _Scopelus_.

_Coccia_ and _Maurolicus_ are two other genera allied to the preceding.

CHAULIODUS.--Body elongate, compressed, covered with exceedingly
thin and deciduous scales; series of luminous (phosphorescent)
spots run along the lower side of the head, body, and tail.
Head much compressed and elevated, with the bones thin, but
ossified, and with the opercular portion very narrow, the
interoperculum being rudimentary. Cleft of the mouth exceedingly
wide, the intermaxillary forming one half of the upper jaw.
Each intermaxillary with four long canine teeth; edge of the
maxillary finely denticulated; mandible with pointed, widely
set teeth, the anterior of which are exceedingly long; none of
the large teeth are received within the mouth. Palatine with a
single series of small pointed teeth; no teeth on the tongue.
Eye of moderate size. Pectoral and ventral fins well developed.
Dorsal fin anteriorly on the trunk, before the ventrals; adipose
fin small, sometimes fimbriated; anal short, rather close to
the caudal, which is forked. Gill-opening very wide, the outer
branchial arch extending forward to behind the symphysis of the
lower jaw; it has no gill-rakers. Branchiostegals numerous.

This genus, of which one species only (_Ch. sloanii_) is known, is generally distributed over the great depths of the oceans, and does not appear to be scarce; it attains to a length of 12 inches, and must be one of the most formidable fishes of prey of the deep-sea.

Allied genera are _Gonostoma_, _Photichthys_, and _Diplophos_, all of which have the teeth of much smaller size.

FOURTEENTH FAMILY--STOMIATIDÆ.

_Skin naked, or with exceedingly delicate scales; a hyoid barbel. Margin of the upper jaw formed by the intermaxillary and maxillary which are both toothed; opercular apparatus but little developed. Gill-opening very wide; pseudobranchiæ none. The eggs are enclosed in the sacs of the ovarium, and excluded by oviducts._

Deep-sea fishes, descending to the greatest depths, characterised by their barbel and their formidable dentition.

Some have two dorsal fins, the posterior of which is adipose; they belong to the genus _Astronesthes_, are the smallest of the family, and frequently met with in the Atlantic.

The others--viz. _Stomias_, _Echiostoma_, _Malacosteus_, and _Bathyophis_, lack the adipose fin, the rayed dorsal being opposite to the anal. Of these the one longest known is

STOMIAS.--Body elongate, compressed, covered with exceedingly
fine and deciduous scales, which are scarcely imbricate, lying
in subhexagonal impressions; vent situated at no great distance
from the caudal fin. Head compressed, with the snout very short,
and with the cleft of the mouth very wide. Teeth pointed,
unequal in size, those of the intermaxillaries and of the
mandible being the longest; maxillary finely denticulated; vomer
with a pair of fangs; palatine bones and tongue with smaller
pointed teeth. Eye of moderate size. Opercular portion of the
head narrow. A fleshy barbel in the centre of the hyoid region.
Dorsal opposite the anal, close to the caudal; pectoral and
ventral fins feeble, the latter inserted behind the middle of
the length of the body. Series of phosphorescent dots run along
the lower side of the head, body, and tail. Gill-opening very
wide. Pyloric appendages none.

Three species are known; beside specimens which were found floating on the surface, others have been dredged from depths varying between 450 and 1800 fathoms.

FIFTEENTH FAMILY--SALMONIDÆ.

_Body generally covered with scales; head naked; barbels none. Margin of the upper jaw formed by the intermaxillaries mesially, and by the maxillaries laterally. Belly rounded. A small adipose fin behind the dorsal. Pyloric appendages generally numerous, rarely absent. Air-bladder large, simple; pseudobranchiæ present. The ova fall into the cavity of the abdomen before exclusion._

Inhabitants of the sea and freshwater; the majority of the marine genera are deep-sea forms. The freshwater forms are peculiar to the temperate and arctic zones of the Northern Hemisphere, one occurring in New Zealand; many freshwater species periodically or occasionally descending to the sea. One of the most valuable families of the class of fishes. No fossils of the freshwater forms are known; but of the marine genera, _Osmerus_ occurs in the greensand of Ibbenbusen, and in the schists of Glaris and Licata; a species of _Mallotus_, indistinguishable from the living _M. villosus_, occurs abundantly in nodules of clay of unknown geological age in Greenland. Other genera, as _Osmeroides_, _Acrognathus_, and _Aulolepis_, from the chalk of Lewes, belong to the same fauna as species of Beryx, and were probably deep-sea Salmonoids.

SALMO.--Body covered with small scales. Cleft of the mouth wide,
the maxillary extending to below or beyond the eye. Dentition
well developed; conical teeth in the jaw bones, on the vomer
and palatines, and on the tongue, none on the pterygoid bones.
Anal short, with less than fourteen rays. Pyloric appendages
numerous; ova large. Young specimens with dark cross-bands
(Parr-marks).

We know of no other group of fishes which offers so many difficulties to the ichthyologist with regard to the distinction of the species as well as to certain points in their life-history, as this genus, although this may be partly due to the unusual attention which has been given to their study, and which has revealed an almost greater amount of unexplained facts than of satisfactory solutions of the questions raised. The almost infinite variations of these fishes are dependent on age, sex and sexual development, food, and the properties of the water. Some of the species interbreed, and the hybrids mix again with one of the parent species, thus producing an offspring more or less similar to the pure breed. The _coloration_ is, first of all, subject to variation; and consequently this character but rarely assists in distinguishing a species, there being not one which would show in all stages of development the same kind of coloration. The young of all the species are _barred_; and this is so constantly the case that it may be used as a generic or even as a family character, not being peculiar to _Salmo_ alone, but also to _Thymallus_ and probably to _Coregonus_. The number of bars is not quite constant, but the migratory Trout have two (and even three) more than the River-Trout. In some waters River-trout remain small, and frequently retain the Parr-marks all their lifetime; at certain seasons a new coat of scales overlays the Parr-marks, rendering them invisible for a time; but they reappear in time, or are distinct as soon as the scales are removed. When the Salmones have passed this “Parr” state, the coloration becomes much diversified. The males, especially during and immediately after the spawning time, are more intensely coloured and variegated than the females; specimens which have not attained to maturity retaining a brighter silvery colour, and being more similar to the female fish. Food appears to have less influence on the coloration of the outer parts than on that of the flesh; thus the more variegated specimens are frequently out of condition, whilst well-fed individuals with pinkish flesh are of a more uniform though bright coloration. Chemistry has not supplied us yet with an analysis of the substance which gives the pink colour to the flesh of many Salmonoids; but there is little doubt that it is identical with, and produced by, the red pigments of many salt- and freshwater Crustaceans, which form a favourite food of these fishes. The water has a marked influence on the colours; Trout with intense ocellated spots are generally found in clear rapid rivers, and in small open Alpine pools; in the large lakes with pebbly bottom the fish are bright silvery, and the ocellated spots are mixed with or replaced by X-shaped black spots; in pools or parts of lakes with muddy or peaty bottom, the trout are of a darker colour generally, and when enclosed in caves or holes, they may assume an almost uniform blackish coloration.

The change of scales (that is, the rapid reproduction of the worn part of the scales) coincides in the migratory species with their sojourn in the sea. The renovated scales give them a bright silvery appearance, most of the spots disappearing or being overlaid and hidden by the silvery scales. Now, some of the species, like _S. fario_, inhabit all the different waters indicated, even brackish water, and, in consequence, we find a great variation of colour in one and the same species; others are more restricted in their habitat, like _S. salar_, _S. ferox_, etc., and, therefore, their coloration may be more precisely defined.

With regard to _size_ the various species do not present an equal amount of variation. Size appears to depend on the abundance of food and the extent of the water. Thus, the Salmon and the different kinds of great Lake-trout do not appear to vary considerably in size, because they find the same conditions in all the localities inhabited by them. A widely spread species, however, like _S. fario_, when it inhabits a small mountain pool with scanty food, may never exceed a weight of eight ounces, whilst in a large lake or river, where it finds an abundance and variety of food, it attains to a weight of fourteen or sixteen pounds. Such large River-trout are frequently named and described as Salmon-trout, Bull-trout, etc. Further, in Salmones, as in the majority of fishes and tailed Batrachians, there is an innate diversity of growth in individuals hatched from the same spawn. Some grow rapidly and normally, others more slowly, and some remain dwarfed and stationary at a certain stage of development.

The _proportions of the various parts of the body_ to one another vary exceedingly in one and the same species. Beside the usual changes from the young to the sexually mature form observed in all fishes, the snout undergoes an extraordinary amount of alteration of shape. In the mature male the intermaxillaries and the mandible are produced in various degrees, and the latter is frequently more or less bent upwards. Hence the males have the snout much more pointed and produced, and the entire head longer, than the females; with the intermaxillary bone the teeth, with which it is armed, are also enlarged, sometimes to four times the size of those of the females. And if this development of the front part of the head happens to be going on while the individual is able to obtain only a scanty supply of food, the usual proportions of the head and trunk are so altered that the species is very difficult to recognise. Barren male fish approach the females in the proportions of the head and body, but hybrid fishes do not differ in this respect from their parents. The abundance or scarcity of food, and the disposition or indisposition of the Salmonoids to feed, are other causes affecting the growth or fulness of the various parts of the body. In well-fed fishes the head is proportionally not only smaller but also shorter, and _vice versa_.

The _fins_ vary to a certain degree. The variation in the number of the rays is inconsiderable and of no value for specific distinction. The caudal fin undergoes considerable changes of form with age, and dependency upon the sexual development. Young specimens of all species have this fin more or less deeply excised, so that the young of a species which has the caudal emarginate throughout life, is distinguished by a deeper incision of the fin, from the young of a species which has it truncate in the adult state. As the individuals of a species do not all attain to maturity at the same age and at the same size, and as mature individuals generally have the caudal less deeply excised than immature ones of the same age and size, it is evident that the variations in the form of the caudal are considerable and numerous, and that it is a very misleading character if due regard be not paid to the age and sexual development of the fish. Further, species inhabiting rapid streams as well as still waters show considerable variations in the form and length of all the fins; those individuals which live in rapid streams, being in almost constant motion and wearing off the delicate extremities of the fins, have the fin-rays comparatively shorter and stouter, and the fins of a more rounded form, particularly at the corners, than individuals inhabiting ponds or lakes. Moreover, one and the same individual may pass a part of its life in a lake, and enter a river at certain periods, thus changing the form of its fins almost periodically.

Finally, to complete our enumeration of these variable characters, we must mention that in old males, during and after the spawning-season, the _skin_ on the back becomes thickened and spongy, so that the scales are quite invisible, being imbedded in the skin.

* * * * *

After this cursory review of variable characters we pass on to those which are more constant, not subject to ready modification by external circumstances; and which, therefore, ought to be noticed in every description of a species of Salmo.

1. _The form of the præoperculum of the adult fish._ The præoperculum is composed of a vertical (posterior) and horizontal (lower) part (limb), both meeting at a more or less rounded angle. The development of the lower limb is a very constant character; in some species (as in the Salmon) it is long, in others (_S. ferox_, _S. brachypoma_) exceedingly short. The adjoining woodcuts will readily show this difference.

In young specimens of all Salmonoids the præoperculum has a very short lower limb; but whilst in some species it lengthens with age, its development in a horizontal direction is arrested in others.

2. _The width and strength of the maxillary of the adult fish._--To show this character in two distinct species, we have given woodcuts of the maxillaries of females (12 inches long) of _S. fario_ and _S. levenensis_ of the same size.

In young specimens of all Salmonoids the maxillary is comparatively shorter and broader, somewhat resembling that of _Coregonus_; yet this bone offers a valuable character for the determination of the young of some species; for instance, in a young _S. cambricus_ it extends scarcely to below the centre of the eye, whilst in _S. fario_ of the same size it reaches to, or even beyond, this point.

3. _The size of the teeth, those of the intermaxillaries excepted._

4. _The arrangement, and the permanence or deciduousness of the vomerine teeth._--In some species the vomer is normally armed with a double or single series throughout life, although, of course, some of the teeth are frequently accidentally lost; in others, these teeth disappear gradually with age, the hinder ones first, so that finally the anterior only remain. In order to ascertain the arrangement of the teeth, it is necessary to remove the gengiva. Frequently the teeth stand in a distinctly double or single series, or they are placed alternately; but frequent irregularities occur which render the character vague, or even unsafe, so that some zoologists have rejected it entirely as unreliable. However, when a greater number of individuals really belonging to the same species are examined, a pretty safe conclusion may be arrived at as regards the arrangement of the teeth.

5. _The form of the caudal fin_ in specimens of a given size, age, and sexual development.

6. _A great development of the pectoral fins_, when constant in individuals from the same locality.

7. _The size of the scales_, as indicated by the number of transverse rows above the lateral line: one of the most constant characters.

8. _The number of vertebræ._--Considering the great number of vertebræ in Salmonoids the constancy of this character is truly surprising. An excess or a diminution of the normal number by two, is of rare occurrence, and generally to be explained by the fact that one vertebra has been abnormally divided into two, two such vertebræ being considerably smaller than the others; or, on the other hand, that two have merged into one centrum, which is then unusually large, and provided with two neural spines. We have seen one case only, in which three vertebræ were united. The number of vertebræ can be easily ascertained in specimens destined for preservation in spirits, by an incision made along one side of the fish, a little above the lateral line.

9. _The number of pyloric appendages._--There can be no doubt that this character may materially assist in fixing a species. We shall see that in some species it varies from 30 to 50; but in others, as in the Salmon and Charr, it has been found very constant (see Fig. 56, p. 131). If unexpected variations occur, their cause may be found in a partial confluence of the cæca, as we have observed that specimens of _S. levenensis_ (a species normally with from 70 to 90 cæca), had those appendages of unusual width when the normal number was diminished.

* * * * *

We have mentioned above that many points in the _life-history_ of the Salmonoids still remain very obscure:--

1. Johnson, a correspondent of Willughby (“Hist. Pisc.,” p. 194), had already expressed his belief that the different Salmonoids interbreed; and this view has since been shared by many who have observed these fishes in nature. Hybrids between the Sewin (_S. cambricus_) and the River Trout (_S. fario_) were numerous in the Rhymney and other rivers of South Wales, before Salmonoids were almost exterminated by the pollutions allowed to pass into those streams, and so variable in their characters that the passage from one species to the other could be demonstrated in an almost unbroken series, which might induce some naturalists to regard both species as identical. Abundant evidence of a similar character has accumulated, showing the frequent occurrence of hybrids between _S. fario_ and _S. trutta_; hybrids between _S. fario_ and species of Charr have been abundantly bred by continental pisciculturists. In some rivers the conditions appear to be more favourable to hybridism than in others, in which hybrids are of comparatively rare occurrence. Hybrids between the Salmon and some other species are very scarce everywhere. The hybrids are sexually as much developed as the pure breed, but nothing whatever is known of their further propagation and progeny.

2. Siebold has shown that some individuals of every species are not sexually developed, and that such individuals differ also externally from those normally developed. However, he appears to have gone too far when he stated that this state of sterility extends over the whole existence of such individuals, and that, therefore, the external peculiarities also remain permanent throughout life. According to Widegren this sterility is merely a temporary immaturity, and a part of the individuals arrive at a full sexual development at a later or much later period than others. To this we may add that many Salmonoids cease to propagate their species after a certain age, and that all so called overgrown individuals (that is, specimens much exceeding the usual size of the species) are barren. Externally they retain the normal specific characters.

The Salmon offers a most remarkable instance of irregularity as regards the age at which the individuals arrive at maturity. Shaw has demonstrated, in the most conclusive manner, that those small Salmonoids, which are generally called Parr, are the offspring of the Salmon, and that many males, from 7 to 8 inches long, have their sexual organs fully developed, and that their milt has all the impregnating properties of the seminal fluid of a much older and larger fish. That this Parr is not a distinct species--as has been again maintained by Couch--is further proved by the circumstance that these sexually mature Parr are absolutely identical in their zoological characters with the immature Parr, which are undoubtedly young Salmon, and that no Parr has ever been found with mature ova. But whether these Parr produce normal Salmon, impregnating the ova of female salmon, or mingle with the River-trout, or whether they continue to grow and propagate their species as fully developed Salmon, are questions which remain to be answered. We may only add that, as far as we know, barren old Salmon are extremely scarce.

3. The question whether any of the migratory species can be retained by artificial means in fresh water, and finally accommodate themselves to a permanent sojourn therein, must be negatived for the present. Several instances of successful experiments made for this purpose have been brought forward; but all these accounts are open to serious doubts, inasmuch as they do not afford us sufficient proof that the young fish introduced into ponds were really young migratory Salmonoids, or that the full-grown specimens were identical with those introduced, and not hybrids or non-migratory Trout of a somewhat altered appearance in consequence of the change of their locality. We have seen the experiment tried at two places in South Wales, and in both cases the Salmon and the pure Sewin died when not allowed to return to the sea. On the other hand, hybrid fishes from the Sewin and the Trout survived the experiment, and continued to grow in a pond perfectly shut up from communication with the sea. In that locality neither those hybrids nor the trout spawn.

4. Although the majority of the mature individuals of a migratory species ascend a river at a certain fixed time before the commencement of spawning, others enter the freshwater at a much earlier period, either singly or in small troops; and many appear to return to the sea before they reascend at the time of the regular immigration. It is not improbable that one and the same individual may change the salt- or freshwater several times in the year. However, this is the case in certain rivers only, for instance, in those falling into the Moray Firth; in others one immigration only is known to occur. The cause of the irregular ascents previous to the autumnal ascents is unknown. A part, at least, of the hybrid fishes retain the migratory instinct; but it is not known whether sterile individuals accompany the others in their migrations.

5. It is said that the migratory species invariably return to the river in which they are bred. Experiments have shown that this is normally the case; but a small proportion appear to stray so far away from their native place as to be unable to find their way back. Almost every year Salmon and Sea-trout in the Grilse-state make their appearance at the mouth of the Thames (where the migrating Salmonoids have become extinct for many years), ready to reascend and to restock this river as soon as its poisoned water shall be sufficiently purified to allow them a passage.

6. There has been much dispute about the time required for the growth of Salmonoids. The numerous and apparently contradictory observations tend to show that there is a great amount of variation even among individuals of the same origin living under the same circumstances, some of them growing much more quickly than others, and being ready to descend to the sea twelve months before their brethren. The cause of this irregularity is not explained. On the other hand, when we consider the fibrous condition of the Salmonoid skeleton, which is much less solid, and more wanting in calcareous substance, than that of the majority of Teleosteous fishes, we shall be quite prepared to adopt the truth of the observation that the young Salmonoids return to the fresh water, after a few months sojourn in the sea, and after having feasted on nourishing Crustaceans, Sand-eels, etc., with their former weight in ounces increased to pounds.

7. Liability to variation in form indicates that an animal can adapt itself to a variety of circumstances; therefore, such species as show the greatest pliability in this respect, are those which most recommend themselves for domestication and acclimatisation within certain climatic limits. Thus, the River-trout or Sea-trout were very proper subjects for those eminently successful attempts to establish them in similar latitudes of the Southern Hemisphere, whilst the attempt of transferring them into the low hill-streams of India ended (as could be foreseen) in a total failure. Those two species must now be considered to be fully acclimatised in Tasmania and New Zealand, and with but little protection may be expected to hold their own in the freshwaters of those colonies. Whether the acclimatisation of the Salmon will be in the end equally and permanently successful, remains to be seen. The true _S. salar_ is not subject to variation, and is very sensitive to any change of external conditions, and to every kind of interference with its economy. The fourth species, with which attempts of acclimatisation in Southern Australia have been made, is a migratory Salmon from the Sacramento river in California. This experiment is still in progress, and believed to be promising of success. It will be a most curious problem to ascertain, how much the original characters and habits of those species will be affected by their transference to so distant a part of the globe. At present it would be too hazardous to offer an opinion on this point, especially as it is a fact that numerous cross-breeds have been introduced into, and reared in, Tasmania, which must more or less interfere with the characters of the pure breeds.

It is apparent, from the foregoing remarks, that the distinction of the various species of Salmonidæ is a matter of considerable difficulty, and that there is scope for great diversity of opinion. At any rate it is only by a close, long-continued study, and constant comparison of specimens of various ages and from various localities, that one is enabled to find a guide through the labyrinth of confusing variations. However, it is a significant fact that the very same characters by which we are enabled to distinguish European species occur again, though in an exaggerated form, in American Salmonoids (which everybody will admit to be of distinct species), and therefore our faith in them necessarily becomes strengthened. In accordance with acknowledged principles in zoology, forms which differ from their congeners by a combination of two or more of constant characters, are to be distinguished under distinct specific names. Most likely they have been derived, at a not very remote period, from common ancestors, but the question of their specific distinctness is no more affected by this consideration than the question whether _Salmo_ and _Coregonus_ are distinct genera. Whenever the zoologist observes two forms distinguished by peculiarities of organisation, such as cannot be conceived to be the effects of an external or internal cause, disappearing with the disappearance of that cause, and which forms have been propagated and are being propagated uniformly through all the generations within the limits of our observations, and are yet most probably to be propagated during the existence of mankind, he is obliged to describe these forms as distinct, and they will commonly be called species.

The species of the genus _Salmo_ are inhabitants of the temperate and arctic zones of the Northern Hemisphere; the species are most abundant in the northern parts of the temperate zone, becoming scarcer beyond the Arctic circle, and in the warmer parts towards the south. The southernmost points in which Salmones are found, are, on the American continent, the rivers falling into the head of the Californian Gulf, and in the Old World the mountain rivers of the Atlas and Hindu Kush. The Salmones from those localities are migratory Trout in the New World, non-migratory and small in the Old. Those species which range to the highest latitudes (lat. 82°) belong to the division of Charr, a group which generally are more intolerant of a moderate temperature, than real Trout. The genus is subdivided into

_a. Salmones_--Salmon and Trout--with teeth on the body of the
vomer as well as its head (see Figs. 289 and 290).

_b. Salvelini_--Charr--with teeth on the head of the vomer only
(see Fig. 291).

Of the host of species (the majority of which is unfortunately very insufficiently characterised) we enumerate the following:--[45]

_a._ SALMONES.

1. _S. salar_ (Salmon; Lachs or Salm; Saumon) (Fig. 6, p. 43).
The Salmon can generally be readily recognised, but there are
instances in which the identification of specimens is doubtful,
and in which the following characters (besides others) will be
of great assistance. The tail is covered with relatively large
scales, there being constantly eleven, or sometimes twelve in a
transverse series running from behind the adipose fin forwards
to the lateral line, whilst there are from thirteen to fifteen
in the different kinds of Sea-trout and River-trout. The number
of pyloric appendages (see Fig. 56, p. 131) is great, generally
between 60 and 70, more rarely falling to 53 or rising to 77.
The body of the vomer is armed with a single series of small
teeth, which at an early age are gradually lost from behind
towards the front, so that half-grown and old individuals have
only a few (1–4) left. The Salmon inhabits temperate Europe
southwards to 43° N. lat., and is not found in any of the rivers
falling into the Mediterranean. In the New World its southern
boundary is 41° N. lat.

2. _S. trutta_ (Sea-trout, Salmon-trout).[46]--Especially
numerous in North Britain.

3. _S. cambricus_ (Sewin).--Wales, South of England, Ireland,
Norway, and Denmark.

4. _S. fario_ (Common River-trout).

5. _S. macrostigma_ (Algeria).

6. _S. lemanus_ (Lake of Geneva).

7. _S. brachypoma._--A migratory species from the rivers Forth,
Tweed, and Ouse.

8. _S. gallivensis_ (Galway Sea-trout).

9. _S. orcadensis._--A non-migratory trout from Lough Stennis,
in the Orkney Islands.

10. _S. ferox._--The great Lake-trout of North Britain, Wales,
and Ireland.

11. _S. stomachicus_ (the Gillaroo of Ireland).

12. _S. nigripinnis_ from mountain-pools of Wales.

13. _S. levenensis_ (Lochleven Trout).

14. _S. oxi_ from the rivers of the Hindu Kush.

15. _S. purpuratus_ from the Pacific coast of Asia and North
America.

16. _S. macrostoma._--Japan.

17. _S. namaycush._--The great Lake-trout of North America.

_b._ SALVELINI: Charr.

1. _S. umbla._--The “Ombre chevalier” of the Swiss lakes.

2. _S. salvelinus._--The “Sælbling” of the Alpine lakes of
Bavaria and Austria.

3. _S. alpinus._--The common Northern Charr, growing to a length
of four feet, and migratory.

4. _S. killinensis._--The Loch Killin Charr, Inverness-shire.

5. _S. willughbii._--The Loch Windermere Charr.

6. _S. perisii._--The “Torgoch” of Wales.

7. _S. grayi._--The “Freshwater Herring” of Lough Melvin,
Ireland.

8. _S. colii._--Charr of Loughs Eske and Dan.

9. _S. hucho._--The “Huchen” of the Danube, growing to the size
of the Salmon.

10. _S. alipes_ from lakes in Boothia Felix and Greenland.

11. _S. arcturus._--The most northern species from 82° lat.

12. _S. fontinalis._--The common “Brook-trout” of the United
States.

13. _S. oquassa._--A lake species from the State of Maine.

_Oncorhynchus_ differs from _Salmo_ only in the increased number of anal rays, which are more than fourteen. All the species are migratory, ascending American and Asiatic rivers flowing into the Pacific. The Californian Salmon (_O. quinnat?_) belongs to this genus.

Other allied genera are _Brachymystax_ and _Luciotrutta_.

PLECOGLOSSUS.--Body covered with very small scales. Cleft of the
mouth wide; maxillary long. Dentition feeble; intermaxillaries
with a few small, conical, pointed teeth; the teeth of the
maxillaries and mandibles are broad, truncated, lamellated
and serrated, movable, seated in a fold of the skin. The
mandibles terminate each in a small knob, and are not jointed
at the symphysis. The mucous membrane in the interior of the
mouth--between the terminal halves of the mandibles--forms a
peculiar organ, being raised into folds, with a pair of pouches
in front and a single one behind. Tongue very small, with minute
teeth, its apical part being toothless; palate apparently
without teeth.

A small aberrant form of Freshwater-Salmonoids abundantly found in Japan and the Island of Formosa.

OSMERUS.--Body covered with scales of moderate size. Cleft of
the mouth wide; maxillary long, extending to, or nearly to, the
hind margin of the orbit. Dentition strong; intermaxillary and
maxillary teeth small, much smaller than those of the mandible.
Vomer with a transverse series of teeth, several of which are
large, fang-like; a series of conical teeth along the palatine
and pterygoid bones. Tongue with very strong fang-like teeth
anteriorly, and with several longitudinal series of smaller
ones posteriorly. Pectoral fins moderately developed. Pyloric
appendages very short, in small number; ova small.

The “Smelt” (_O. eperlanus_) is common on many places of the coasts of Northern Europe and America. In the sea it grows to a length of eight inches; but, singularly, it frequently migrates from the sea into rivers and lakes, where its growth is very much retarded. That this habit is one of very old date, is evident from the fact that this small freshwater form occurs, and is fully acclimatised in lakes which have now no open communication with the sea. And still more singularly, this same habit, with the same result, has been observed in the Smelt of New Zealand (_Retropinna richardsonii_). The Smelt is considered a delicacy in Europe, as well as in America, where the same species occurs. Two other allied genera, _Hypomesus_ and _Thaleichthys_, are found on the Pacific coast of North America, the latter being caught in immense numbers, and known by the name “Eulachon” and “Oulachan;” it is so fat, that it is equally used as food and as candle.

MALLOTUS.--Body covered with minute scales, which are
somewhat larger along the lateral line and along each side
of the belly; in mature males these scales become elongate,
lanceolate, densely tiled, with free projecting points, forming
villous bands. Cleft of the mouth wide; maxillary very thin,
lamelliform, extending to below the middle of the eye. Lower jaw
the longer, partly received between the maxillaries. Dentition
very feeble; the teeth forming single series; only the teeth on
the tongue are somewhat larger and disposed in an elliptical
patch. Pectoral fins large, horizontal, with broad base. Pyloric
appendages very short, in small number; ova small.

The “Capelin” (_M. villosus_) is found on the Arctic coasts of America and of Kamtschatka. It is caught in immense numbers by the natives, who consume it fresh, or dry it for use in the winter. Its length does not exceed nine inches.

COREGONUS.--Body covered with scales of moderate size. Cleft of
the mouth small; maxillary broad, short or of moderate length,
not extending behind the orbit. Teeth, if present, extremely
minute and deciduous. Dorsal fin of moderate length; caudal
deeply forked. Ova small.

The majority of the species, of which more than forty are known, are lacustrine species; and comparatively few are subject to periodical migrations to the sea, like _Salmo_. They are confined to the northern parts of temperate Europe, Asia, and North America. Their distribution is local, but sometimes three and more species are found in the same lake. They abound in every lake and river of the northern parts of North America, and are known by the name of “White-fish.” They are of vital importance to some tribes of the native population. The European _C. oxyrhynchus_ is as much a marine as a freshwater species. In the British Islands several small species occur, viz. _C. clupeoides_, the “Gwyniad,” “Schelly,” or “Powen” from the great lakes; _C. vandesius_, the “Vendace” of Lochmaben; and _C. pollan_, the “Pollan” of the Irish lakes. The latter is brought in quantities to Belfast market during the season, that is, at the time when it rises from the depths of Lough Neagh to deposit its spawn near the shore. Thomson says that in September 1834 some 17,000 were taken there at three or four draughts of the net. Some of the species of the continent of Europe and America attain to a much larger size than the British species, viz. to a length of two feet.

THYMALLUS.--Principally distinguished from _Coregonus_ by its
long many-rayed dorsal fin.

“Graylings”--five species, inhabiting clear streams of the north of Europe, Asia, and North America. The best known are the “Poisson bleu” of the Canadian voyageurs (_Th. signifer_), and the European Grayling (_T. vulgaris_).

SALANX.--Body elongate, compressed, naked or covered with
small, exceedingly fine, deciduous scales. Head elongate and
much depressed, terminating in a long, flat, pointed snout. Eye
small. Cleft of the mouth wide; jaws and palatine bones with
conical teeth, some of the intermaxillaries and mandibles being
enlarged; no teeth on the vomer; tongue with a single series of
curved teeth. Dorsal fin placed far behind the ventrals, but in
front of the anal; anal long; adipose fin small; caudal forked.
Pseudobranchiæ well developed; air-bladder none. The entire
alimentary canal straight, without bend; pyloric appendages
none. Ova small.

This small, transparent, or whitish fish _(S. chinensis_) is well known at Canton and other places of the coast of China as “White-bait,” and considered a delicacy. It is evidently a fish which lives at a considerable depth in the sea, and approaches the coast only at certain seasons.

Finally, this family is represented in the deep sea by three genera, _Argentina_, _Microstoma_, and _Bathylagus_, of which the two former live at moderate depths, and have been known for a long time, whilst the last was discovered during the “Challenger” expedition in the Atlantic and Antarctic Oceans at depths of 1950 and 2040 fathoms. As _Argentina_ is sometimes found in the North Atlantic, and even near the British coasts, we give its principal characters.

ARGENTINA.--Scales rather large; cleft of the mouth small;
intermaxillaries and maxillaries very short, not extending
to below the orbit. Eye large. Jaws without teeth; an arched
series of minute teeth across the head of the vomer and on the
fore part of the palatines; tongue armed with a series of small
curved teeth on each side. Dorsal fin short, in advance of the
ventrals; caudal deeply forked. Pseudobranchiæ well developed.
Pyloric appendages in moderate numbers. Ova small.

Four species are known, of which _A. silus_ and _A. hebridica_ have been found occasionally on the North British, and, more frequently, on the Norwegian coast. The other species are from the Mediterranean. Attaining to a length of 18 inches.

SIXTEENTH FAMILY--PERCOPSIDÆ.

_Body covered with ctenoid scales; head naked. Margin of the upper jaw formed by the intermaxillaries only; opercular apparatus complete. Barbels none. Gill-openings wide. Adipose fin present._

One genus and species only (_Percopsis guttatus_); interesting as having the general characters of Salmonoids, but the mouth and scales of a Percoid. Freshwaters of the northern United States.

SEVENTEENTH FAMILY--HAPLOCHITONIDÆ.

_Body naked or scaly (cycloid). Margin of the upper jaw formed by the intermaxillary; opercular apparatus complete. Barbels none. Gill-opening wide; pseudobranchiæ. Air-bladder simple. Adipose fin present. Ovaries laminated; the eggs fall into the cavity of the abdomen, there being no oviduct. Pyloric appendages none._

Freshwater-fishes which represent the Salmonoids in the southern hemisphere. Two genera only are known. _Haplochiton_ (Fig. 104, p. 250) abundant in lakes and the streams falling into the Straits of Magelhæn and in the rivers of Chile and the Falkland Islands. It has the general appearance of a Trout, but is naked. _Prototroctes_, with the habit of a _Coregonus_, scaly, and provided with minute teeth; one species (_P. maræna_) is common in South Australia, the other (_P. oxyrhynchus_) in New Zealand. The settlers in these colonies call them Grayling; the Maori name of the second species is “Upokororo.”

EIGHTEENTH FAMILY--GONORHYNCHIDÆ.

_Head and body entirely covered with spiny scales; mouth with barbels. Margin of the upper jaw formed by the intermaxillary, which, although short, is continued downwards as a thick lip, situated in front of the maxillary. Adipose fin none; the dorsal fin is opposite to the ventrals, and short, like the anal. Stomach simple, without blind sac; pyloric appendages in small number. Pseudobranchiæ; air-bladder absent. Gill-openings narrow._