Chapter III (1)

5. Globular eyes. Cambrian, in the youngest zone, the Olenus schists. The oldest at present known oculate trilobite Eurycare is found in the second division of these schists.

In the lowest Lower Silurian division, the Ceratopyge limestone, Euloma and Ceratopyge occur as the last survivors of the blind, partially ridge bearing genera. The Trinucleidæ and Ampyx belong to another group of trilobites.

Even among other exclusively Lower Silurian genera, in which the plurality of the species is oculate, there are species entirely blind. So with Illænus, in which genus Dr HOLM has not found any eye in Ill. Angelini, I. leptopleura and Ill. cæcus. The free cheek in these three species is much narrow, as the facial suture lies near the margin of the head.

=The eyes of the Trilobites.=

If, as is probable, to judge by the conformity of their cornea with that of recent crustacea, the trilobites like these were provided with crystalline cones beneath the corneal lenses or facets, only the latter have been preserved in a fossil state. Although the crystalline cones in consequence of their solid consistence might have been petrified as well as the cornea, they must, imbedded as these tiny cones lie, entirely wrapped up in delicate tissues, fall away and be lost, when the dissolution of the dead body had set in. Consequently the curious appendages on the inferior side of the lenses in Dalmanites vulgaris (Pl. III f. 50) or Phacops quadrilineata (Pl. V fig. 38) can noways be considered as belonging to the original structure of the eye, apart from their great dissimilarity with anything appertaining to the eyes of the Arthropoda. The cornea on the contrary cohered with the integument of the body, and it has been well preserved in a great number of trilobites.

I subdivide the trilobites in respect to the form of their eyes in the following manner:

=I. Genera with compound eyes.=

1. _With prismatic plano-convex cornea facets._

Acidaspis, as a transitional form to the next group.
Asaphus.
Bumastus.
Cyphaspis.
Dysplanus.
Encrinurus.
Illænus.
Megalaspis.
Nileus.
Phillipsia.
Niobe.
Proetus.
Ptychopyge.
Symphysurus.

2. _With round or biconvex transversally elongate lenses._

Acerocare.
Bronteus.
Chirurus.
Ctenopyge.
Cyrtometopus.
Eurycare.
Peltura.
Sphærophthalmus.

=II. Genera with aggregate eyes of biconvex lenses.=

Acaste.
Chasmops.
Dalmanites.
Phacops.

=III. Genera with isolated eyes, one or several stemmata at the extremity of a straight facial ridge.=

Harpes.
Harpides.
(?) Trinucleus in the larval state.

Only a few authors have before now occupied themselves with the intimate structure of the trilobite eye. PACKARD gave in 1880, in the »American Naturalist» a note on the structure of the eye of trilobites (p. 503). There are some rough and inexact sketches of the eyes of Limulus and Asaphus, and although he seems to have known the beautiful researches of GRENACHER he still »claims that the trilobite eye was organized on the same plan as Limulus». This statement is altogether wrong, and as I hope to show the trilobites have had eyes entirely different from that of Limulus and instead agreeing with those of the Isopoda and perhaps also with a few other Crustacea. In 1889 J. M. CLARKE published an account[28] on the »Structure and development of the visual area in the trilobite Phacops rana GREEN». The aggregate eye described by him are of the type forming my third group. His holochroal division embraces my first and second groups and the schizochroal my third.

[Footnote 28: Quart. Journ. of Morphology, vol. II p. 253.]

The latest contribution to the knowledge of these eyes is found in EXNER'S »Physiologie der facettirten Augen von Krebsen and Insecten», 1891, where he gives good figures of the lenses of Phacops fecundus, pl. II figs 18, 19. He says that the palpable difference in the structure of these eyes and those of Limulus point to a change in the function of these eyes.

I. =Compound eyes.=

=1. Eyes with prismatic, plano-convex lenses.=

A pellucid, smooth and glossy integument, a direct continuation of the common test of the body covers the corneal lenses, quite as is the case in so many of the recent crustacea. In the plurality it is, however, difficult to discern the lenses from the outside.

The lenses, as seen in a vertical section of the eye of Asaphus expansus, (pl. I fig. 12), are columnar prisms, like the pillars of basalt, attaining a length of 0,2 mm and at the point where the eye joins the test of 0,3 mm. At their interior extremity they have a breadth of 0,066 mm. On that point the surface is convex and at the exterior surface plane. They are closely packed and in a transverse section resemble a pavement of regular hexaeders. But they also assume other shapes and become rhombs or even quadrates, as seen in a specimen of Asaphus fallax (pl. I fig. 18), where the hexaeders and quadrates lie side by side without transitional forms. As a rule the lenses become more and more irregular in the vicinity of the surrounding frame or near the suture, nearly blotted out, as it were, and without any definite border line mingled with the confused, spongy mass that like a belt or a frame surrounds they eye in Asaphus and is sharply limited from the other part of the free cheek. This remarkable zone which is almost only present amongst the Asaphidæ (Asaphus, Megalaspis, Ptychopyge, Isotelus) retains in a confused manner somewhat of the prismatic structure of the eye as shown in the section (Pl. I, fig. 11, b). The eye of Bumastus also is environed by a similar zone, with a structure like that of the eye (Pl. II fig 35, 41).

In an undetermined species of Asaphus the lenses, although somewhat apart, are of an elongated hexaedric outline, which passes into a regular circular one farther away and on the surface of the eye they are slightly convex (Pl. I figs 27-29). In other genera belonging to this group the shape of the lenses are like those of Asaphus, so for instance in Illænus (I. chiron and I. Esmarki) and in Niobe. In Dysplanus centrotus they are shorter and broad, and their interior or lower surface strongly convex. It is likewise so in Nileus, where Nileus armadillo has an exceedingly thick exterior integument above the lenses. Such an integument has in a still higher degree increased in Bumastus sulcatus so as to exceed in thickness the stratum of the corneal prisms and it may in fact be doubted if the eyes of this species ever were able to function as visual organs. Proetus nearly resembles Bumastus in the thickness of the integument covering the prismatic lenses, which are interiorly convex, with a diameter of 0,03 mm.

In all genera belonging to this group a horizontal section gives the image of the hexaeders as in Asaphus with some change to squares or rhombs.

In scrutinizing a horizontal, somewhat extensive section of an eye in this group of trilobites, it will be perceived, as for instance in the figures (Pl. V fig. 16, 22) that the regular and evidently homogenous and intact prismatic lenses by and by have been altered and in a part of the section, a little distant from the intact ones disintegrated in their interior, showing various aspects of alteration. I cannot but think that this is a destruction which has set in long after the fossilization. It has revealed certain states of the intimate structure, certain delicate details, that now with an astonishing regularity come in sight and probably also lie hidden in the intact prisms. In the specimens of Asaphus, which we have studied, the alteration has taken the shape of a concentric stratification forming the body of the prisms, which is well discernible in a horizontal section, but not easy to detect in the longitudinal one (Pl. I figs 9-10, 11). It is likewise so in Niobe. In the other genera again the decomposition makes the prisms look like empty tubes in which a few irregular traverses and trabecular remains of their solid mass radiate towards the interior. They thus assume the aspect of a composite coral with its septa in the calicles (Pl. VI fig. 31). This is also evident in Nileus palpebrosus and Dysplanus.

=2. Eyes with biconvex lenses.=

The surface of the eye is, as in Chirurus glaber ANG., a mass of contiguous hemispherical lenses, probably once covered with a membrane, as is still to be seen in well preserved specimens of Bronteus laticauda. Both in Chirurus and Bronteus the lenses seen vertically are globular and ordinated beside each other either continuous or separated only through a faint dividing line. In a horizontal section passing right through the point of contact they show the common hexaedral shape and when somewhat corroded the interior radiate structure also comes forth, the radii directed towards a little black point in the centre. The lenses of the Brontei have the same stellate structure as in Bumastus (Pl. II fig. 7). In Cyrtometopus the lenses are in size the fourth of those in Chirurus and they form an extremely thin stratum in strongest contrast with the adjoining cheek, which surpasses them more than six times in thickness (Pl. III fig. 19). The lenses of Cyrtometopus are more flattened and irregular than in the former genera. The free cheek around the eyes does not form a border zone, somewhat imitating the eye structure as in Asaphus, but is more compact, composed of vertical elements which give to the test of the trilobites in general a tendency to split up in vertical prisms.

Of a peculiar interest are the eyes in the oldest of all oculate trilobites, at present with certainty known, Eurycare, Peltura, Sphærophthalmus and Ctenopyge.

Of these genera Eurycare is the oldest (see table p. 22). Amongst the many free and detached cheeks only a single, very little one has been found with the eye ball fixed. It seems to be of the same structure as in Sphærophthalmus. In Sphærophthalmus and Ctenopyge the eye globes are enormous, considering the size of the cheek in which they are set and occupy more than a third of the length of the free cheek (Pl. III f. 26, 31). They are hemispheric, blackish and glossy, more so in the former genus. The spheroidal lenses, projecting on the surface, are in Ctenopyge larger near the facial suture and small at the opposite side where the eye is fixed in the free cheek. For the rest, in both genera (Pl. III fig. 34) the lenses form a thin stratum, where they in a vertical section lie elongated, flattened and biconvex, slightly joined with each other at the point of contact. The fine form which they exhibit reminds of the lenses of Sphæroma.[29] They are in diameter thrice as long as they are high. Seen in a horizontal section passing through the point of contact they show hexaeders with a curiously jagged outline (Pl. III fig. 33).

[Footnote 29: BELLONCI Atti dei Lincei. Memorie, vol. X, 1881, Sphæroma, pl. II fig. 11.]

Peltura which is coeval with these, has a narrow semiglobose visual field (Pl. III figs 35-41), the superior surface of which is quite smooth and evenly rounded. On its interior side there stand out, somewhat distantiated, in a low relief semiglobular facets, quite as regular incrassations of the cornea, thus not forming free lenses, but rather reminding of the for the rest differently formed quasi-lenses of Limulus. In a vertical section they appear as the inferior moiety of real ovate lenses (Pl. III f. 40-41).

The much younger Acerocare has a similar cornea. A very little specimen, the head of scarcely more than one millim. in length, retains both eyes, of which one shows the slightly convex lenses and the other a cast of the interior side as in Peltura. These both genera should in consequence of their peculiar limuloid cornea be ranged for themselves apart from the real lenticulate genera, but any material sufficient for doing this properly, is at present not at hand.

II. =Aggregate eyes.=

These are found solely in the family of the Phacopidæ, unless the Lichadidæ were also provided with this sort of eyes, but we have had no opportunity to study them. It seems, however, not likely that they had aggregate eyes. BARRANDE has represented them quite as finely reticulate as the eyes of any Asaphid. We have sectioned and figured the eyes of Dalmanites vulgaris and D. obtusus from the Silurian of Gotland and found that these have truly aggregate eyes, each consisting of a regular biconvex lens, lying enclosed in a socket of its own and covered by a cornea of its own. The distance between the eyes is much variable and in a few instances they are nearly contiguous. Extremes are seen on pl. III figures 43, 47. The lenses are comparatively large, and have always had a covering membrane, though this in many instances has been lost. This membrane which is an immediate continuation of the general integument of the body covers the lenses all round their superior moiety. In its prolongation downwards between the lenses (Pl. VI fig. 3, 4) it is free from the contact with them and hangs alongside and around much incrassated, so as to take in a section a lengthened lancet like shape. It lies thus alongside the other interstitial test, and is like this perforated by longitudinal canals. In a horizontal section taken a little below the surface it encircles the lens as a wall like ring (Pl. VI fig. 1, 2). In a vertical section the lenses lie in direct contact with the cheek without any intervening zone and the cheek has the structure so common amongst the trilobites, being perforated by vertical tubes going straight down from the surface (Pl. III fig. 44, Pl. VI f. 5).

In Dalm. vulgaris and also in Phacops quadrilineata there is as already before mentioned a peculiar structure beneath the lenses, consisting of narrow, threadlike, straight lines, twice as long as the lenses (Pl. III 49-50, pl. V fig. 38). In a horizontal section they are found to be irregular prisms closely packed. It can not be any structure peculiar to the eyes or the lenses, rather some parasitic growth added since the death of the animal. The lenses are in several specimens composed of clear calcareous spar. In others again they have been filled with a dark muddy calcareous rock excepting in the lower moiety where there is left a residue of the white spar, having in all lenses assumed a regular shape which I consider as organic (Pl. VI fig. 5). This spar covers the whole bottom and its upper rim is incrassated and bent inwards. In horizontal sections this residue is a whitish ring close inside the interior ring wall (Pl. VI f. 2). I would suggest that this curious conformation is due to the original structure of the lens, supposing that it in these crustaceans has been built upon the same plan as in several other Arthropoda. In Cymothoa[30] and in Sphæroma[31] for instance the lenses are built up of thin strata, which are parallel with the convex outside, so that on the inferior surface of the lens they are arched downwards and on the superior side upwards, being not strictly concentric. In the spiders they are constructed upon this same plan[32] perhaps more evidently. If now in Phacops the lens consisted of such semiconcentric strata and the upper moiety has been destroyed, the rest must have taken the shape as we find it. It is moreover peculiar that the destruction has been exactly similar in all lenses of that specimen. Can it be due to the circumstance that the power of resistance in the inferior strata has been greater?

[Footnote 30: BULLAR Philos. Transact. 1878, pt. II, pl. 46, fig. 12.]

[Footnote 31: BELLONCI Atti dei Lincei, Memorie X 1881, pl. II, fig. 11.]

[Footnote 32: See GRENACHER pl. II f. 18 Epeira.]

In Phacops quadrilineata the lenses are more elliptic than in the former. On their interior surface beneath the spiny tufts mentioned large hexaedral prisms of clear calcareous spar issue, one prism for each lens (Pl. V fig. 38), having thus a very deceptive appearance, but no doubt of inorganic origin with the lenses as a basis for their crystallization, quite as in the Cystoids where the interior often is converted into a mass of crystalline prisms, issuing from the interior surface of the plates.

III. =Genera with stemmata and ocelli.=

In a little group that has retained larval or ancestral characters during a great part of the palæozoic period, the genus Harpes stands as a type. It has ranged from the oldest Lower Silurian, if we join the related Harpides, to the middle Devonian. From near the top of the glabella, though not so much forward as the facial ridge of the blind trilobites (Olenus, Liostracus etc.) a straight ridge of much varying length stands out on both sides and at its extremity two or three globular stemmata with glossy surface lie encased. Probably this ridge has the same origin as in the Olenidæ, the more so, as there are indications of an extensive circulatory system. On plate IV fig. 18-19 the right hand ocelli of Harpes vittatus BARR. from Lochkow, Bohemia, are represented. They are two, lying isolated near each other, quite globular with circular outline, smooth and glossy as to exhibit a shining surface. Their size is 0,4 mm in diameter. Being cut vertically in the direction of the longitudinal axis of the head they resemble elongated hemispheres, convex on the exterior surface, slightly concave on the interior. The test of the head lies between them as a saddle and covers them only partially and on the outer sides they lie with their margins encased in the head shield. Seen in thin sections of the right lens and magnified the whitish mass is traversed by vertical, blacker streaks, standing somewhat radiating towards the sides and cancellate. The other lens has a horizontal row of black dots. All this is probably not of any structural value, only due to later changes.

The remarkable genus Harpides from the lowest Lower Silurian belongs also to this group and has beside the fixed peduncle a peculiar elongated ridge going from the eyes to the lateral margins of the head[33], a ridge which is also present in some of the true Harpes.

[Footnote 33: Harpides breviceps cannot belong to this genus and is rather related to Erinnys SALTER, as also MATTHEW holds it.]

As BEECHER has shown[34] the larva of Trinucleus possesses quite the same transverse ridge with intumescent eyelike extremities, and although the smallness of the specimens has not permitted to ascertain the presence of a true eye, it may be apposite to suppose it on the homology with the eyes of Harpes, a genus with which Trinucleus is related. But as well known, in the adult Trinuclei there is no trace of these ocular ridges nor of real eyes so that BEECHER in his paper »Blind Trilobites»[35] numerates Trinucleus amongst these. Out of the nine Scandinavian species of Trinucleus no less than seven have a well marked little tubercle on each side of the glabella placed exactly on the same spot where the larval Trinuclei had their much larger eyes placed. In this case it may be allowed to suppose that the tubercles are the direct successors of the larval eye and that they are true ocelli. REEDE seems to be willing to regard them as possessing a visual function.[36] BEECHER[37] holds the eye nodules in the larva and the ocelli in the adult to be identical.

[Footnote 34: Structure and appendages of Trinucleus.]

[Footnote 35: Geol. Magazine 1898. pp. 439, 493, 552.]

[Footnote 36: l. c. p. 447.]

[Footnote 37: l. c. p. 309.]

But, as BARRANDE has shown,[38] there is a certain species of Trinucleus, the larva of which wants a facial ridge and eyes, as there also are several adult forms without ocelli. These have remained on a much ancestral stage, while the larva with eyes are more highly developed in such species, where the adult have been subject to a retrograde development. Ampyx and Dionide, though completely blind, evidently belong to this group, and once, as is to be hoped, larval forms may be discovered showing their development. In a certain way Arethusina shows characters proper for this group, in having the straight ocular ridge, quite as in Harpes, but eyes of the reticulate type and probably prismatic. It thus like Harpes conserved an ancestral characteristic long periods since it had disappeared in most of the other genera.

[Footnote 38: Sys. Sil. de Bohéme I, pl. 30, figs 41-50.]

Of the groups, in which JOH:S MÜLLER[39] long ago classified the Crustacean eyes, his second »Hauptgruppe» (»Aggregate von einfachen linsenhaften Augen») and the fourth »zusammengesetzten Augen ... facettirte Hornhaut» the former corresponds with my third and the latter with my two first divisions. In so far as the cornea and its facets or lenses are to be regarded, there is the greatest analogy with the Isopoda. In vertical sections of Sphæroma we have the same sort of elongated, flattened biconvex lenses as in Sphærophthalmus and others. Since GRENACHER and EXNER and others have published their excellent works on the eyes of the Arthropoda, there can be no foundation for speaking of the resemblance of the trilobitic eye with that of Limulus, as this genus stands completely isolated amongst all Arthropoda in that respect. There is, as stated above, a certain resemblance between the cornea of Peltura and that of Limulus, but this is not yet ripe for a discussion. Nor is there any evidence for correlating the eyes of the trilobites with the eyes of the Phyllopoda. BERNARD thinks that the so called eye of the Paradoxidæ has been formed upon the same plan as that of Apus. There is nothing to prove this hypothesis that the facial ridge or any part of it ever had been a visual organ, and the evidence at hand rather tends in a contrary direction.

[Footnote 39: In Merkels Archiv 1829 p. 46 and in Treviranus Zeitschrift für Physiologie Bd IV p. 97.]

There are signs of long physiological and anatomical efforts to prepare the development of the eyes on the free cheek, as revealed through the long series of blind trilobites. A system of radiating blood vessels, similar to those described above as covering the inside of the head in some older genera, all issuing from the scallop in the free cheek, where later the eye had to find its place, have left their stamp, their mark on the surface of the free cheek. They attest the great vital activity which was so intense at the point were the eye was to be formed. We give the figures of two such cheeks of different types. One from Parabolina spinulosa (pl. V fig. 31) is the more common, where six or more isolated trunks radiate from the semilunar ridge round the indenture and subdivide in branchlets which cease near the lateral margin of the cheek. It may be that it is an annular vessel near the indenture that feeds them all and that this probably is in connection with the great central circulatory system. In Olenus (pl. V fig. 29) the vessels are partly anastomosing and form a reticulate system and they are studded with minute wartlets. Another sign, which may be taken as a preparation, is the elevated rim around the scallop, which is so prominent in several of the Cambrian genera, but which does not embrace any facet bearing cornea.

From what has been stated above the following conclusions have been arrived at.

1. The plurality of the genera living during the Cambrian period were blind and it was first at the close of that period, in the Olenus schists, that genera with real visual organs appeared. There may have been oculate trilobites earlier, as Solenopleura, but we know nothing of their eyes.

2. The primordial glabellar pleuron which was metamorphosed into a facial ridge is no visual organ. It is in the Olenidæ nothing but the elevated line made in the test by the subjacent main trunk of the circulatory system. It swells out in a node, »palpebral lobe», but not before the facial suture has been formed. In the genera where there is no facial suture, there is no node. In the Paradoxidæ where the ridge is of a different origin, there is no node, though there is a suture.

The four types of eyes in the trilobites have probably succeeded one another in the following chronological order:

1) with stemmata or ocelli; 2) biconvex or lentiform; 3) prismatic; 4) aggregate. The oldest known representatives for each type are for 1) Harpides rugosus in the Ceratopyge limestone of the Lower Silur., for 2) Eurycare, in the Cambrian Olenid schists, division 2, for 3) Megalaspis, in the Ceratopyge limestone of the Lower Silurian, for 4) Phacops in the Lower gray Orthoceratite limestone. The eyes of the trilobites show the greatest conformity with those of the recent Isopoda.

The most perfect eyes amongst all the trilobitic eyes may be those of the Phacopidæ, which are also geologically the youngest, the least developed again those of the Proetidæ or rather of the Bumasti. The great thickness of the cornea in these must have weakened their power of vision and they had probably only a faint perception of light.

=On the maculæ of the hypostoma.=

We shall now turn our attention to the visual organs which LILJEVALL discovered on the hypostoma of Bronteus. In doing this, we may bear in mind, that the genera in which we really have found lenses on the maculæ are relatively few, but that we shall review the maculæ in all genera, which we have been able to examine, and try to show that even most of these may, although in an inferior degree, have acted as visual organs. At the same time some more details shall be given about the cephalic eyes for comparing them with the hypostomic ones.

=Acidaspis= MURCH.

The hypostoma is of a peculiar type, deviating from that of the other genera, squarish or rectangularly transverse, entirely without terrace lines and no maculæ proper. BARRANDE has given no less than nine different samples of these hypostomas.

=Acidaspis crenata= EMMR.

(Pl. I figs 1-6.)

There are certainly no maculæ of the same sort as in so many other trilobites and the interior side of the hypostoma does not bear the least traces of impressions which might be taken as the reverse of the maculæ and still more less as muscular impressions. In fact, the total absence of such in Acidaspis militates against the interpretation of the macula impressions as muscular scars. When the maculæ fail, also the so called muscular impressions fail. Moreover we are able to see still clearer in this matter through what we know about the structure and position of the extremely similar hypostoma of Apus, of which we have given a description in the end of this memoir. This skeletal part of Apus is without any connection with the surroundings, excepting at its anterior margin, and is consequently movable in a direction outwards and upwards. Along that anterior margin it is fixed to the outside of the ventricle through three pair of muscles, three muscles on each side of the margin. ZADDACH De Apodis cancriformis anatome (pl. II fig. XIV p. 68).

For the rest the marks of the attachment of the muscles are as a rule in the Crustacea, at least those of the head, elevated small platforms, so in the Trilobites, of which I have excellent specimens in a Bumastus and others. On the inside of the head of Limulus they are faintly elevated patches. The small hollows on the inside of the hypostoma formed by the maculæ are indeed the sockets in which the soft parts of these more or less developed hypostomic eyes were sheltered. But there is still a feature in the hypostoma of Acidaspis which merits our attention and which perhaps may have a significance akin to that of the maculæ.

The hypostoma is square, with two short pointed wings, one on each side of the slightly bent anterior margin and likewise two smaller ones at the corners of the posterior margin. A groove follows on a short distance the lateral margins and the posterior margin and disappears a little below the anterior one. In the same direction, distally, though a little more inside and unconnected with them there are two small grooves, the bottom of which consists of a shell substance of different colour and structure than the other parts. Having been a little ground and seen in transmitted light it exhibits the shape of a club and a homogenous yellow spot, tapering posteriorly and swelling out distally (Pl. I fig. 4). It must be left an open question whether these maculæ share in the nature of visual organs as the quite different maculæ of the other trilobites, but it may be possible that it is so. It must, however, be remarked that there are two types of hypostoma in the genus Acidaspis as shown by the illustrations of BARRANDE. One has the small grooves, possibly all sheltering the claviform maculæ, disposed as in the now described A. crenata. This group embraces five species of the Bohemian Silurian formation. The other group of three species again has the hypostoma of the same quadratic or rectangular shape, but the two short grooves, which may be expected to contain the maculæ, are placed midways between the anterior and the posterior margins, nearly as the maculæ bearing grooves of other trilobites. We have however not had material for pursuing our researches in this genus, the other species of the Swedish Acidaspidæ being unknown as to their hypostoma.

It may here be added an observation concerning the ornamentation of the exterior surface of the hypostoma of Ac. crenata. It is covered by a great number of diminutive circular or oblong wartlets occupying the whole surface excepting the lower third of the central field just above the posterior groove which is smooth. These wartlets seen through transmitted light (Pl. I fig. 5) show in their interior something like a peculiar black spiculum rising from a bifid rootlet and confined within the wartlet and not extruding from it. In a longitudinal section (Pl. I fig. 6) the spicula perforate the wartlets reaching through their whole length. As in Calymmene these interior pseudo-spicula are tubes, filled with iron-pyrites. It is probable that these tubes were once bearing setæ and quite as in Apus formed a fur of bristles.

The structure of the cephalic eyes (Pl. 1 f. 1-2) is prismatic, but the separate prisms are rather short and broad. Their lower or interior end is convex. The separating lines between the single prisms are not always distinct.

=Agnostus= DALM.

Pl. 1 fig. 7.

Agnostus glandiformis ANG. Although there is not the slightest evidence of eyes in this the largest of its genus, nor any free hypostoma hitherto has been found, we may here give a little account of our researches into this species. The scantiness of material forms a chief obstacle to our knowledge. Only three entire, rolled up specimens have been found, and it is by sectioning and preparing such that any hope can be entertained to gain reliable results.

A rolled up specimen from Andrarum Scania was sectioned lengthwise. The tail-piece closed tight against the head-shield, so there had been little chance for foreign matter to penetrate into the interior which, however, is filled. Close below the cephalic shield there is a remarkable structure, mostly resembling an elongated intestine with swellings joined by more narrow ducts and anteriorly the coherence is interrupted. As in crustaceans and Arthropods in general the stomach and the intestine are situated on the dorsal side of the body, there is nothing unlikely in assuming that this in reality may be the remnants of the intestine. This may also be compared with the observation made by VOLLBORTH in his memoir »Ueber die mit glatten Rumpfgliedern versehenen Trilobiten», 1863, p. 46, tab. 1 fig. 12 where the heart-tube probably is delineated. BARRANDE has also given figures of what he considers as the intestine in Trinucleus.

Below this organ in Agnostus, there lies a section of a vaulted calcareous plate with its convexity turned against the dorsal side of the head shield, that is to say quite the reverse what might have been expected if it had been the hypostoma in its true position. This may, however, not be any objection against considering it as a sectioned hypostoma, loosened from its connection with the cephalic shield, disturbed in its original position and turned round, when the shell became filled with mud. At the distal end there is a much distinct duplicature.

=Asaphus= BRONGN.

The wellknown hypostoma of this genus has the same characteristic shape, though more pronounced, as in Ptychopyge, with its posterior margin deeply indented, so as to form two large, pointed lobes. There is properly only one median field, surrounded by flat, lateral borders, continuing down into the posterior lobes, from which it is separated through a shallow groove. The macula are situated in this groove, on each side of the inferior border of the central field. They are more or less prominent, but whatever their form may be, their surface is always entirely smooth, lying well circumscribed amidst the surrounding terrace lines.

The following list enumerates all the species in this genus, of which previous authors have delineated the tubercles, though they in the descriptions only in very few instances have mentioned their presence.

_A. acuminatus_ NIECZKOWSKI, »Zusätze zur Monogr. der Trilobiten 1859», tab. I, f. 6. Maculæ most prominent, but no mention made of them in the description.

_A. (Isotelus) canalis_ J. HALL, Pal. N. Y. I, pl. 4 bis, f. 18-19. Though this and a following species probably on account of the deviating conformation of their body, belong to a different genus or subgenus, I mention them along with the Asaphi, as there is the greatest similarity in their hypostoma. Fragment of an interior cast; still more strange is the fragment showing part of the interior surface and the duplicature.

_A. expansus_ L. BR. I pl. VII f. 3, BR. II pl. I f. 2, 2 c. In the two first figures there are no tubercles nor maculæ marked, in the last figure there are distinct tubercles.

_A. fallax_ A. pl. XXVIII f. 3, c. BR. II pl. I, f. 3, both nearly congruent.

A. (Isotelus) gigas J. HALL, Pal. N. Y. 1, pl. 60, f. 7 g, pl. 66 f. 5, the inside of an entire, uncommonly large hypostoma gives a general good view with the macula; which are large and evident. He mentions the maculæ as »two circular spots». »These probably indicate the points for the attachment of muscles and tendons upon the inside,» he adds.

_A. ingens_ BARR. NOVÁK II, pl. I, f. 7, a good figure showing two semiglobular maculæ. BARRANDE'S figures pl. 33, f. 7, 8 are not distinct.

_A. ludibundus_ TQT. BR. II, pl. 1, f. 7. Two semilunar sulci on the cast of the interior surface, being only the posterior borders of the maculæ which are a little more raised and distinct than the rest.

_A. nobilis_ BARR. pl. 32, f. 6. Indistinct traces of maculæ. In the figure 6, pl. 31 representing the hypostoma of a young specimen, there are no tubercles at all. If the figures were to be relied upon, it might be assumed, that the tubercles appear at a more mature age.

_A. Powisii_ SALTER, pl. 23, f. 6. Two semilunar, narrow tubercular maculæ, their interior apices converging towards the anterior border of the hypostoma.

_A. raniceps_ A., pl. XXVIII, f. 2 c., BR. II, pl. 1, f. 4. A good figure.

_A. raniceps_ var. _maxima_ BR. II, pl. 1, f. 6. Semilunar impressions on a cast and thus far incomplete.

_A. striatus_ BOECK. To this belongs probably A. expansus M. SARS »Ueber einige neue oder unvollständig bekannte Trilobiten» in Isis 1835 p. 333, and especially p. 340 and the following (Bemerkungen über die untere Seite von einigen Trilobiten). SARS there gives a very good description of the exterior side, where he also mentions the two maculæ, pl. IX, f. 9 a, b. (»2 kleine Knoten»), BR. I, pl. VIII, f. 4 a, and BR. II, pl. I, f. 9. These latter figures differ in so far, that the former has the maculæ excavated or rather tubular below, in the latter again they are regular.

_A. trinucleorum_ BR. II, pl. I, f. 16 a large specimen with linear maculæ arranged rectangularly in respect to the longitudinal axis of the hypostoma and both on the same level.

_A. tyrannus_ MURCH. SALTER in Mem. Geol. Survey, Brit. Foss. Dec. II, 1849 pl. V, f. 4, excellent figure. On p. 2 he says: »there is an oval circumscribed tubercle at the origin of each (fork) most distinct on the inner surface». This figure is again reproduced in the »Monograph», pl. 22, f. 6, where it is said »Two linear tubercles with their interior apices converging towards the posterior margin of the hypostoma».

_Asaphus_ sp., probably A. raniceps, POMPECKJ, Trilobitenfauna Ost- and West-Preussens, taf. VI, f. 7. p. 80 »an seinem Hinterrande liegen zwei kleine, flache Höckerchen, die als Reste des Hinterlappens aufzufassen sind»(!?).

We shall now describe the Asaphi which we have been able to examine more in detail.

=Asaphus expansus= L.

Pl. I figs. 8-17.

In the numerous specimens of which we have examined the hypostoma, there are always small, nearly circular maculæ which by their lighter colour are marked out from the surrounding smooth space of the inferior sinus of the lateral grooves where they are situated. They do not rise at all above the surrounding surface from which they are separated only by a fine, scarcely perceptible line. On the interior surface of the hypostoma they are better seen, and the enclosing line is deeper and more distinct. Their surface is there quite as smooth and even as on the exterior side of the hypostoma. They do not consequently in the least manner resemble muscular impressions as observable on the inside of the glabella of other trilobites. These are on the contrary elevated above the surface and finely striated in various ways.

The peculiar nature of these macula is revealed through the various sections we have made. In pl. I fig. 15 a vertical section across the entire hypostoma is represented. The sectioned, dark maculæ (a) lie on both sides of the faintly curved central field, in the sinus of the lateral grooves, and the strongly developed terrace lines continue sidewards just a little on the interior side, where the duplicature turns round. As seen in a magnified vertical section (fig. 16) the maculæ consist of horizontal, whitish, straight lines, probably lines of successive growth, and these are crossed rectangularly by more irregular whitish lines separated from each other through dark spaces. The whole thus gains the aspect of a dark surface cancellated by white lines. This reticulated or spongious macula is enclosed as to its superior as well as to its inferior part in the compact and homogenous test of the hypostoma and occluded from the influence of the light. Its value as a visual organ consequently is insignificant. In a horizontal section (fig. 17) the structure is still more bewildering. There the whole macula is a confused, spongious white mass with dark spaces between the white meshes, and only at one side, the left one, some obscure indications as of polygones are visible. It would indeed have been impossible to interprete what this means, had not the study of the cephalic eye given a clue thereof.

The eye of this species is covered with a delicate perfectly smooth and glossy integument (f. 12) which is reposing immediately on the prismatic lenses of the cornea. In a few instances, depending on the colour or the state of preservation of that integument the lenses are indistinctly translucent. Owing to their state of preservation their aspect is greatly variable. They are in many instances, as seen in fig. 9, six-sided or some rhombic, foursided or even quadratic (f. 8, 10). They are all of the same length, 0,2 mm. along the surface proper of the eye, but are lengthened to 0,3 mm. towards the border of the eyes, f. 12, which will be described further on; they are of equal breadth, amounting to 0,066 mm. Their inferior ends are slightly convex or nearly plane when well preserved, else, when as often is the case, disintegrated as to be scooped out and vaulted. Their interior structure, as revealed by sections, shows a cylindrical core, f. 8-10, composed of concentric strata. This cylinder fills nearly the whole interior space of the prisms, there being, however, in many instances a compact dark mass between the cylinder and the walls of the prisms. There are also sections in which the prisms are filled with a uniformly black mass without any concentric structure. In longitudinal sections the outlines of the individual prisms are not clearly discernible (f. 11). There are longitudinal, white lines of varying thickness with lateral irregular offshoots, which may join with those from opposite walls, and give the interior a sort of spongious or cancellate appearance.

Towards both sides of the ocular surface, towards the superior and inferior side, a change sets in as to the shape of the lenses, as best seen in horizontal sections (figs. 8, 10) they are lengthened and become more and more indistinct, and at last in the upper and lower marginal zones pass over into a reticulate, spongious mass, which seen in a longitudinal section presents almost the same aspect as in the regular prismatic surface of the eye. Fig. 11 _b_, _a_ being the visual field. It is, however, more densely reticulate, but a prismatic arrangement is quite as much evident as in the ocular surface proper. In the rule the passage from the prismatic surface to the reticulate is gradual, the prisms becoming by and by irregular in their outline and diminishing in size (fig. 12, _a_ the eye proper, _b_ the border zone), but there are also instances where the distinction between these two fields is sharp and without any gradual transition. In the inferior reticulate zone there are generally some oblongue, funnelshaped pits. I am uncertain whether they are to be regarded as regular parts of the eye structure or rather as burrows of some parasite. They do not continue deep down.

If we now compare the reticulate zone of the eye with the maculæ of the hypostoma, for instance the vertical section fig. 16 with fig. 11 _b_, we find the most complete identity in structure. In the same manner the horizontal section of the border zone of the eye, fig. 8 _a_, fig. 10 _b_, and of the macula, fig. 17 are similar. The same chaotic, spongious mass in both, with some tendency to form prisms more evident in the longitudinal sections, where the same reticulate structure with predominant white, longitudinal streaks is so palpable. That there is a complete identity in structure between the two, the macula of the hypostoma and the border zones of the eye, is as evident as anything can be, but as to the functional identity or what this function may have been it is difficult to decide anything with certainty, at least it seems to me that the capacity of vision must in both have been far more restricted than in the eye proper. They rather give the impression as of rudimentary visual organs.

=Asaphus raniceps= DALM.

Pl. I f. 23-26.

The maculæ of the hypostoma are placed obliquely in the saline manner as in the preceding species. They are more prominent and the oblong macula, with the longest diameter of 1,2 mm. is on the top of a little mound and surrounded by a fine, elevated marginal line (fig. 23). Its somewhat convex surface is entirely smooth and if sectioned horizontally exhibits the same sort of spongy texture as A. expansus (pl. I f. 24). In a vertical section (fig. 25) the macula does not occupy so large a space as in As. expansus, but rather lies as a lenticular disk in the hypostomic test closer to the superior surface than the inferior. With sufficiently high power the same sort of pillars, divided by horizontal strata is seen. Upon the whole the vertical section is not so clearly developed. The reticulate zone of the eye is more definitely separated front the prismatic zone than in A. expansus. There can be no doubt that there is a correspondence in this species between the structure of the macula! and the spongious zone of the eyes.

=Asaphus cornigerus= SCHLOTH. (A. Kowalewskyi LAWROW).

Of this strange species with its enormous eyestalks, more than 2 centimeters in length I have through the kindness of Akademiker FRIEDR. SCHMIDT in St. Petersburg had occasion to study some specimens.

The rather large maculæ, obovate-circular, are like those of several other Esthonian species oriented inwards and upwards instead of inwards and downwards as in the Swedish species described. As the specimens have suffered through corrosion of the surface there are only faint traces of a marginal line. Their microscopic structure is badly preserved, and they look pale and transparent with only few indications of the spongious texture. The lenses of the cephalic eye are nearly square prisms sometimes with a slight approach to hexaedral pillars. There is no clear transition into a spongious or reticulate border zone. But this may depend upon the bad preservation.

=Asaphus fallax= DALM.?

Pl. I fig. 18-22.

It is very difficult to distinguish this species or rather variety from A. expansus, but if we have found genuine specimens, there are some points in the shape of eyes and maculæ which make it different. The hypostoma (f. 21) is rather more broad and the duplicature is large with an upturned margin. The little macula is placed on the top of a smooth rounded elevation and enclosed by an elevated rim.

The cephalic eyes are rather short, regular hexaedral prisms (f. 18-20) and as shown in fig. 18 change into irregular squares near the border of the eve.

=Asaphus= sp.

from Brunsby kanal, Segerstad parish, isle of Öland. The maculæ are large pale whitish and of an uncommonly fine-meshed reticulation, well limited from the surrounding hypostoma.

=Asaphus= sp.

Pl. I fig. 27-30.

from the islet of Sandö, north of Gotland. Although we cannot give any account of its hypostoma, not having had sufficient material, the structure of the eyes is so peculiar that it seems worthy of being recorded. The integument is extremely thin and transparent and the subjacent lenses are clearly seen, and through their impact on the thin integument they make this to stand out in a very low relief above them (fig. 27). These lenses are uncommonly short, forming at the surface rather oblong, slightly hexaedral prisms with a narrow interspace between them. In a section lower down they have the shape of hollow, white rings filled with black mud and in a longitudinal section the white walls of the lenses look like short pointed spikes and interiorly they are completely empty. We here find also the same gradual change from regular cones to the spongious border zone as in the above mentioned species f. 29. The border zone is finely reticulated.

=Asaphus (Isotelus) gigas= J. HALL.

Pl. II figs. 1-3.

The enormous hypostoma of this giant resembles in a high degree that of the Asaphi, but is at the same time the most evident verification of the experience that the hypostoma _per se_ cannot be regarded as the sole criterion for determinating the generic affinities of different species. The whole structure of this trilobite in other respects gives it a quite independent position, distinct, from Asaphus.

We have had at our disposal several specimens of the large hypostoma, the dimensions of the largest, fragmentary in its anterior margin, being as follows: breadth 48 mm. length 41 mm., probably 47 mm. when entire, breadth of each of the posterior lobes at their bases 19 mm., length of the same 23 mm. The maculæ which are placed on the flat surface of the hypostoma without being at all elevated, are prominent through their great size and their pale, whitish colour. They are somewhat oblongue having their longest axis directed inwards and downwards. They attain a diameter of four millimeters. Horizontally sectioned, f. 2, they show the spongious texture and vertically the quasi prismatic reticulate structure richly developed, f. 3. The pillars are very distinct and continue without interruption all through the macula and in the interstices there are traverses joining between two pillars or partially filling the darkish interspaces, thus giving the whole the aspect of some »tabulate» coral sectioned. By the inspection of the horizontal section alone, presenting the irregular spongy texture it would have been impossible to imagine the ordinated arrangement which the vertical section reveals to us. The eyes consist of regular hexaedral prisms, and there is a very sharp boundary line between them and the spongious zone, which is very narrow and without distinct separating lines joins with the cephalic test. For the rest there are in all probability at least two different species sent from America under the name of Isotelus gigas, of which only the largest, almost like a Homalonotus, and the hypostoma of which has been partly delineated in this memoir, seems to be the real one. They differ both in the shape of the hypostoma as well as in other respects.

=Barrandia= MAC COY.

According to SALTER Monogr. pl. 19 fig. 9 his B. Portlocki, of which there is a fragmentary and broken hypostoma, shows feeble traces of oblique macula, nearly in the fashion of Asaphus. In the description at page 139 is told about »the usual pair of tubercles».

=Bronteus= GOLDFUSS.

BARRANDE delineates the maculæ on the hypostoma of

_B. palifer_, Tab. 45 fig. 17,

_B. planus_, Tab. 48 fig. 7, and

in the supplementar volume of

_B. rhinoceros_, Tab. 9 f. 16,

_B. furcifer_, Tab. 11 fig. 16, but there is not the slightest indication of granulation on any of them, nor is there in the descriptions, generic or specific, the least mention made of the tubercular maculæ. It is also remarkable that in the works of ANGELIN and NOVÁK where several hypostomas belonging to species of this genus are delineated, not a single one shows these tubercles. We shall now continue the descriptions of the Swedish Brontei, already begun with Br. polyactin in the introductory part of this memoir.

=Bronteus irradians= LINDSTR.

Pl. II figs. 4-5.

has a hypostoma that much resembles that of Br. polyactin. In its general shape it is similar and the two concentric grooves with the two maculæ placed in the same way, just below the superior groove. These maculæ are much larger than in Br. polyactin, nearly thrice their size. They are also more ovate or rather like a bean, the smooth surface is larger and the granulated spot restricted to a more narrow space forming an oblique patch. The granules or lenses are also individually larger than in the allied species, double their size or 0,06 millimeters. We have not succeeded in making sections of the cephalic eye nor of the maculæ.

=Br. platyactin= ANGELIN.

Pl. II f. 14-19.

The hypostoma has a transversally triangular form, and is divided only in two fields through a shallow semicircular groove near the posterior margin. The two maculæ are situated above the groove near its superior sinuses. They are elongated, fig. 17, elliptic with the narrow pointed end directed outwards and the broad rounded end inwards. The chief surface is scooped out as a shallow depression. The granulated spot is situated on the broader end and covering it completely. The relatively large lenses are arranged in five regular rows, the uppermost one being the longest. On the interior surface of the hypostoma there are the corresponding sockets of both maculæ with smooth surface. The horizontal sections of the granules figs. 18, 19 present the image of white rings in close contact, without, however, to occasion a prismatic structure, a dark interspace lying between each ring. These lenses are filled with a dark mass, and in some the same sort of radiated structure is perceptible as in the lenses of the cephalic eye. In horizontal sections of the cephalic eye the lenses approach the polyedral shape. In another section near to the surface of another specimen the lenses are decidedly hexaedral. The vertical sections, fig. 14, reveal their real nature as lenses where they lie as a string of beads with a dark nucleus enclosed within a thin whitish shell. They are covered by a thin membranous lining. When seen in transmitted light the lenses proper are dark, and the shell white and in reflected light the lenses are lighter than the rest.

=Br. laticauda= ANGEL.

Pl. II figs. 6-13.