Chapter III (2)

Lower Silurian from Dalecarlia. The hypostoma, f. 10, is of a broad clypeate shape, the anterior margin rounded without any large projecting wings. On the exterior surface there are two grooves parallel with the rounded inferior margin. As in Br. polyactin the two elliptic tubercles are situated on the inferior edge or slope of the upper groove and they are deepened by a shallow depression as in Br. platyactin. The granulated spot situated along the posterior margin of the macula deviates much in shape from that of the other species. It consists of a long and narrow stripe ending in a fine point outwards and widening inwards, where it is rounded, forming thus a claviform, curved elevation. Around the granulated area the maculæ are quite smooth, and show in a horizontal section an irregular structure of tiny black dots nearly resembling the structure of the hypostomic shell. We have not been able to obtain any good vertical section, but by casts of the interior side of the tubercles it is found that the granules form polyedric facets like those of the eyes though perhaps not so regular. There is a specimen with the granules intact, and the polyedric shape is then not so distinct.

Sections of the eyes elucidate the structure, which partially is obscure in other species of Bronteus. There is a thin membranous coating covering the subjacent well formed lenses (fig. 8). When this membrane in some instances has been peeled off the lenses lie bare (fig. 9). These have a dark nucleus and in some instances it seems as if there were two. In a vertical section they look spheroidal, in a horizontal section again they are polyedric, especially when taken somewhat below the surface or near the middle line (fig. 6). In fig. 7, some lenses are delineated in a horizontal section, highly magnified and the corallian appearance is evident. The lenses are well separated by distinct lines, and from their inner tubes whitish reticulations radiate towards the centre.

If we now compare these sections, where the lenses are so distinctly seen, with the more obscure sections of other Brontei, it is evident that the dark points in them are nothing but transformed and deteriorated centres of the lenses. In Br. platyactin they are a little more distinct than in the other species. And it is with these that the granulated spots on the maculæ of the hypostoma are most concordant.

=Bronteus= sp. indet.

Pl. II figs. 28-30.

This species, of which only the hypostoma is known, found at Lansa, Gotland, is likely, to judge by that, to be nearly related to Br. polyactin. The shape of this hypostoma is exactly the same and the maculæ are placed in the same position, but they are larger, a little sunk on the blind surface and the group of the lenses is different. Its superior margin forms an ingoing arch and the lenses themselves are not convex, but rounded or slightly polyedric, and separated through thick interspaces.

=Bumastus= MURCHISON.

The only recorded hypostoma with maculæ belongs to B. insignis HALL as described by SALTER Monogr. p. 208 pl. 27 fig. 7. He says: »A pair of compressed tubercles occurs at the lower third: they are transverse-ovate, and more than their own diameter apart.» This is nearly in concordance with what is seen on the species to be described below.

Although it can in reality be said that the species of Bumastus are the most common of all trilobites in the Silurium of Gotland, in so far that their head pieces and pygidia are met with everywhere in the limestone rocks of the island, the find of entire specimens or of detached hypostomas is amongst the rarest events when collecting there and amongst thousands of fragments, in several places entirely filling large portions of the limestone rock, there have in the Swedish State museum been acquired only three or four hypostomas, which we are now going to describe.

At first we must point out a certain resemblance which prevails between the hypostomas and maculæ in Bumastus and Bronteus. Compare for instance Bum. sulcatus with Br. platyactin and Br. laticauda. The general form is nearly the same in both and the shape of the maculæ almost identical but for the complete want of granulations on those of Bumastus. To this resemblance must be added that there are, as known, forms of trilobites which by some authors have been regarded as Brontei and by others, again, ranked amongst the Illænidæ.

Three species of Bumastus have been recorded as found in the Gotland strata, but of one of these no hypostoma is known. Besides, to judge by the head shields and hypostoma, there are a few new species. To begin with the most common

=B. sulcatus= LDM.

Pl. II figs. 33-40.

The hypostoma is broadly shield-formed and the anterior margin is most characteristic being in its central part elongated in a short evenly rounded projection. The side wings are triangular acuminate, bent a little backwards and towards the interior surface. There is only a single rounded field passing into an evenly bent border, in front of which the projecting, elliptic maculæ lie. A second pair of wings stretches backwards near the posterior margin. We have not obtained any sections of the maculæ.

The cephalic eyes figs. 36-38 are covered with an uncommonly thick stratum of homogenous shell, one and a half times as thick as the subjacent lenses. These are comparatively broad, above passing indistinctly in the covering stratum, below, at their basis, convex. In a horizontal section figs. 33, 34 they exhibit the common hexaedral or pentaedric aspect, their exterior sides being white and confluent without any separating lines. Their interior is dark and from the polygonal sides white lines go in giving the whole the appearance as of a starry, composite coral with its septa. The correspondence in structure with Bronteus laticauda pl. II fig. 7 is striking. A narrow space separates the surface of the eye from a nearly similar patch below the eve on the cheek 35 _b_, 38 _b_. But there the position of the composing strata is singularly reversed and the spongious or pseudo-prismatic stratum lies close to the outside and the homogenous stratum on the inside. The white longitudinal lines are well marked out and regular in the vertical section, fig. 38 _b_, indicating the prismatic structure most clearly.

=B. barriensis= MURCH.

Pl. II figs. 31, 32.

The hypostoma fig. 31 is of so transverse a shape that the breadth much surpasses the height, the former being 6 millim., while the latter attains only 3,5 millim. The anterior margin is so much arcuated and sloping backwards that it forms an obtuse angle. The side wings are triangular and acuminated and from them a thick pad runs backwards and forms the posterior margin. Below the largest field of the exterior surface the maculæ rest on a ridge having a much elongated, vermiform shape and sunk in the surface (fig. 32) a little more than three times longer than broad. In its shape it approaches much to the hypostoma of Bum. insignis HALL as delineated by SALTER pl. 27 fig. 7.

In another, undetermined Bumastus (pl. II figs. 47, 48)[40] the hypostoma (from Klints, Othem, isle of Gotland) is of a more elongate obovate form, the anterior margin elevated, rounded, the terrace lines faint, the tubercles are larger than in the former Bumasti, oblong, curved, beanshaped, evenly rounded and smooth.

[Footnote 40: Probably the same as Bum. sulcatus pl. XII fig. 12 in my paper on the Trilobites of Gotland 1885.]

Another of the new species is one that LILJEVALL has found at Korpklint near Wisby (Pl. II figs. 41-46). The hypostoma is more elongate than transverse with the anterior margin regularly arcuated, by far not so prominent as in B. sulcatus and the former. By a shallow groove the exterior surface is divided in two fields, the anterior one the largest and covered with a few, distantiated, concentric terrace lines. The posterior field is smooth, nearly even and the maculæ lie just in the groove, with their axis oblique to the longitudinal axis of the hypostoma, beanshaped and smooth. The cephalic eyes show in a horizontal section hexaedral white walls enclosing a dark space in which some indistinct radii emanate, fig. 41 _a_. In the vertical sections the prisms are narrow and elongate and the integument by far not so thick as in Bumastus sulcatus, fig. 42, 43. The border zone of the eye in this (fig. 41) as well as in B. sulcatus, as seen in horizontal sections, agrees with that of the Asaphidæ.

=Bumastus= sp. inlet.

Pl. III fig. 1, 2.

In a detached piece of limestone, found in Gotland, at Norderstrand, near Wisby, two hypostomas and a free cheek lay embedded along with a coral of the genus Acantholithus and a Bronteus probably belonging to a new species. As the coral has not been found above the uppermost beds of the Lower Silurian it is probable that the trilobites also are derived from the same horizon.

The hypostoma, figured pl. III fig. 1, is broadly tongueshaped, with a short, blunt, triangular wing on each side of the anterior margin. The exterior side is nearly completely occupied by a single large field, which is convex and decorated by sparse terrace lines. Below this a smooth, nearly even plane, reaching to the inferior, elevated border of the hypostoma. The two tubercular macula' are situated exactly on the boundary line between the convex and the plane field. They are elongately ovate, pointed outwards, rounded inwards, with smooth and glossy surface. That portion, on which in the species of Bronteus the granular spot rests, is transversally wrinkled by some faintly elevated, sigmoid lines (fig. 2). It would have been of great interest to investigate the interior structure of these curious maculæ, but the scarcity of the material has prevented our doing so.

The other hypostoma, pl. III fig. 3-5, is larger and has the terrace lines more distantiated and forming more open curves, but for the rest it is of the same shape as the smaller, so that we may not ascribe these small deviations to a specific difference. The maculæ are more elongated, narrow, curved tubercles, thus differing from the next preceding. A section running through its left tubercle does not show any structure, but the shell of the maculæ is extremely thin, thus contrasting strongly with the surrounding thicker shell, fig. 4 _a_. The cephalic eye, again, remaining on the free cheek found in the same piece of limestone and probably belonging to the same specimen, has its surface well preserved and exhibits semiglobular lenses in low relief, fig. 5.

=Calymmene= BRONGN.

In the following species of this genus hypostomic tubercles have formerly been recorded.

_Cal. Blumenbachi_ SALTER Monogr. pl. 8, fig. 9. The identity of the species is dubious.

_Cal. duplicata_ SALTER Mon. pl. 9, fig. 22. The tubercles rather more linear and approximated than in the subsequent. SALTER calls them »two small transverse lobes ... forming a nearly continuous ridge».

_Cal. tuberculata_ ANGEL. Tab. XIX fig. 5 c from the inside.

It is remarkable that the eyes or that part of them where the cornea and the lenses should be found, has been destroyed and entirely lost in all specimens of the Calymmene known to us, many hundreds having been searched and every one with a lacuna on that spot. It must have been of an excessive thinness. We know only of one instance, the Bohemian Calym. Arago, in which BARRANDE has seen the eye intact and provided with a small number of about eleven sphærical lenses.

=Calymmene intermedia= LDM.

Pl. III figs. 6, 7.

The clypeiform hypostoma is divided in two fields, one anterior larger, in the centre elevated in a short, blunt knob, and a posterior, bearing the two elevated rounded maculæ, just below the semicircular groove which separates it from the anterior field. These fields are surrounded by a large border which is posteriorly emarginated through a short rounded sinus. The surface is entirely covered by small semiglobular wartlets perforated by a straight pore or duct, which being filled with a black mineral, probably iron pyrites, gives a peculiar and characteristic aspect to the sections. In the same manner the whole mass of the hypostoma is pierced by straight, black lines, also filled up pores, starting from the outside and ending before attaining the inner surface.

The maculæ are prominent, separated from the hypostoma through narrow, distinct grooves, ovate, with the pointed end directed obliquely outwards, against the margins of the hypostoma. They are covered with wartlets and leaving only a little oblong spot free, quite smooth, this being consequently the macula proper. But in a longitudinal section there is not the least distinction between this macula and the other hypostoma, only, that it is free of pores.

=Calymmene tuberculata= BRÜNNICH.

Pl. III figs. 8, 9.

We have succeeded to prepare a horizontal section of the macula. This is oval, perfectly homogenous and without any structure proper, showing only indistinctly a mottled medley of pale, brownish spots in the clear mass. There is in both maculæ a small dark, angular spot placed in their inferior part, fig. 9.

=Centropleura= ANGELIN.

This author has delineated most distinct maculæ in Centr. Steenstrupi pl. III fig. 4, b. two, small elongate darkish spots, near the posterior margin. I have not seen any specimen of this trilobite and have consequently not been able to follow up ANGELIN'S observation.

=Chasmops= M'COY.

Pl. III fig. 10.

In his work on the Russian Phacopidæ FRIEDR. SCHMIDT has given the figures of Chasmops Eichwaldi and Ch. Wesenbergensis showing, as it were, patches of black maculæ. But as there is no mention of them, and as they are quite unlike the macula? which are described here, I am in doubt of their real nature. We have figured a macula from the hypostoma of a Chasmops, probably Ch. macroura, from a drift block near Rostock. It is elongate and narrow, more than four times longer than broad. Its smooth surface is a little scooped out and no structure is observable.

=Chirurus= BEYRICH.

We find the hypostomic maculæ annotated and delineated as occurring in the following species.

_Ch. bimucronatus_ SALTER pl. 5, fig. 5 as two narrow elongated pits, surface seems intact. No mention of them in the letter-press.

_Ch. exsul_ SCHMIDT pl. VI fig. 9. Indications of the oblong, narrow maculæ.

_Ch. macrophthalmus_ SCHM. pl. VII, figs. 1 c, 2. Faint indications of narrow pits.

_Ch. Quenstedti_ BARR. Tab. 42 fig. 3. Maculæ near lower border as two narrow slits.

_Ch. spinulosus_ SCHM. pl. VII fig. 10. The lateral pits well visible.

_Ch. tumidus_ SCHM. pl. VIII, fig. 22. The pits are short, but present.

The exterior surface of the hypostoma is entirely covered with small wartlets, closely set, of different size in the various species, and spread between them lie a great number of elongate smooth spots dispersed, on the interior side of the hypostoma showing an excavated surface (pl. III fig. 16).

=Chirurus (Cyrtometopus) clavifrons= DALM.

Pl. III figs. 17-21.

The shape of the hypostoma is shown in fig. 21 pl. III. It is regularly shield-formed with posterior margin rounded. A shallow groove running parallel with the margins forms a large, faintly elevated border. The anterior margin projects on both sides in a short, backwards directed, broad and flat wing, hollowed out with a pit on the front side, and there is a posterior lateral wing, midways between the anterior and posterior margin short, blunt, directed obliquely backwards. The whole exterior surface is covered with diminutive tubercles, the above mentioned smooth spots interspersed on the central disk. A little below the middle there are two lengthened pits, so shallow that they often are not discernible. In a specimen of Chirurus spinulosus NIECZKOWSKI from Estland there are two, almost 3 mm. long dots, one on each side above the maculæ as the fig. 21 shows and in several others of this generic group there are also indications of similar. I cannot compare these dots with anything more than the lengthened spots visible in Acidaspis crenata.

The maculæ are wanting in this species, but the cephalic eyes are well developed. A section of them fig. 19 shows the enormous difference of the shell in the eye and the surroundings where the shell surpasses it many times in thickness. The eye consists in fact only of the ovate, beadlike lenses of which a string is seen sectioned in the figure mentioned. More enlarged (fig. 20) a nucleus is visible in each lens, and in a horizontal section (fig. 18) a little below the surface, where they are more pressed against themselves, they have a polygonal shape. The figure 17 seems to represent lenses that have been much changed interiorly, having only a narrow zone left of the primary structure.

=Chirurus glaber= ANGELIN.

Pl. III fig. 11.

As a sample of the shape of the surface of the eyes in this genus, we have given a figure, showing the small rounded lenses.

=Chirurus ornatus= DALMAN.

Pl. III figs. 12-14.

The cephalic eyes have globular lenses, of more than double the size of those in Chir. clavifrons. The difference between the lenses and the surrounding shell amounts at the highest to thrice the former, while in Ch. clavifrons it is at least seven times as much. In a horizontal section the lenses have a little, darkish irregular nucleus surrounded by a radiated structure (fig. 12).

The maculæ (fig. 14) are slightly concave, oblong, smooth, surrounded by comparatively large granulations.

=Chirurus speciosus= DALM.

Pl. IV fig. 1.

The exterior surface of the hypostoma is sparingly covered by granules of larger size than in other Chiruri. The maculæ as seen in a cast are prominent near the posterior border, sunk in their centre and surrounded by a narrow distinct border line. In a horizontal section it as not been possible to detect any peculiar structure, only a dark border surrounding the interior clear surface.

_Chirurus spinulosus_ NIECZKOWSKI.

Pl. III fig. 15.

We have had the interior surface of a hypostoma at our disposal from the lower Siluria, of Esthonia (Kuckers C^2) belonging to Dr. G. HOLM. The macula are well preserved, standing out black on the white surface, oblong, 1,5 millim. in greatest length (fig. 15). There are indications as of a great number of small lenses on these maculæ. Remarkable are the two elongated spots, almost 3 mm. in length, above these maculæ, of which already mention has been made under Ch. clavifrons.

=Chirurus= sp. indet.

Pl. III fig. 16.

The hypostoma of an undescribed species from Öland. The maculæ are lengthened, ellipsoid and smooth tubercles lying in a groove. The finely granulated surface of the hypostoma shows rare smooth spots interspersed, being on the inside smooth pits, from which again in casts of the hypostoma larger tubercles are moulded. Of the same nature are those that are visible on the nucleus of Chir. speciosus and others. There are still some undescribed Lower Silurian species, for instance one nearly related to Ch. conformis with tubercular maculæ. A large hypostoma from the Leptæna limestone of Dalecarlia, 33 millim. in length, has a longitudinal macula, 2 millim. in length, as seen on the inside of the fragment.

On pl. VI fig. 10 a little hypostoma is delineated that shows some resemblance with that of the Chirurus, but probably belongs to some other, unknown genus. It is Upper Silurian, found at Mulde in Fröjel, Gotland. It is of an elongated ovate shape, with the anterior border faintly arched, the anterior wings broad, truncate. A narrow elevated border surrounds the lateral and posterior margins. The somewhat vaulted surface is covered by fine granulations, and the macula, situated on equal distance from the anterior and posterior margins are ovate, smooth and directed obliquely inwards. There are two minute pointed processes on each side of the lateral borders and one on each side of the pointed posterior margin.

=Ctenopyge= LINSN.

=Ct. spectabilis= BR. (I, pl. XII, fig. 12 a).

A small lengthened, apparently smooth hypostoma with two globular tubercular maculæ near the posterior border. No mention made of them in the description of BRÖGGER. Cambrian. I am not, however, quite sure whether this really belongs to a Ctenopyge, as it rather more resembles the type of a Peltura as shown in Pelt. scarabæoides. As Ctenopyge is so nearly related to Sphærophthalmus it could be expected to see its hypostoma of the same type and the maculæ entirely wanting. The eyes of this genus have been described above at page 29.

=Cybele= LOVÉN.

=Cyb. bellatula= DALMAN.

Pl. IV fig. 2.

We give a new figure of the hypostoma to complete that of FR. SCHMIDT, pl. XIII fig. 9. The anterior wings are larger and obtuse, excavated below and the border rim is not so distinct all round as he has figured. There are two pair of oblique, lengthened grooves above each other. The opposite do not, however, join to form a coherent groove across the median field of the hypostoma, which is level between them. No maculæ. Nearer the anterior border close below the wings, there is a small roundish, dark spot. The surface of the hypostoma is granulated.

=Cyphaspis= ANG.

=Cyph. elegantula= ANG.

Pl. III figs. 22-25.

The hypostoma is elongated, anterior margin evenly arched, anterior wings triangular, lateral margins faintly curved as well as the posterior margin, at both sides of which there is one diminutive spine. The posterior wings are short forming a spiny process from the lateral margin. The surface of the hypostoma is smooth with a shallow transverse groove on the inferior moiety and two corresponding lateral impressions below it, one on each side. The elevated lateral borders are longitudinally striated by a few terrace lines. There are no maculæ visible. The eyes consist f short hexagonal prisms (figs. 22, 23) nearly resembling those of Proetus.

=Dalmanites= EMMRICH.

In this large genus the hypostoma has been figured in a great number of species. But it is only in the following that we, at least in the figures, if not in the descriptions, are able to detect the maculæ.

_D. atavus_ BARR. Suppl. pl. 5 fig. 14.

_D. Calypso_ J. HALL, vol. VII, pl. XI A. fig. 21 p. 66 »Postero-lateral pits, moderately strong and elongate». There are casts of the two well distinct maculæ.

_D. caudatus_ BRÜNN.

a. FORBES and SALTER in British Organic remains, Dec. II pl. 1, fig. 3 give a good figure, showing the oblique pitlike maculæ. On page 2 is told that »a pair of lateral strong indentations indicate a second furrow above» (»the transverse furrow between the tip»).

b. SALTER, pl. 3, figs. 7, 8, bad figures, the description verbally the same as above. What ANGELIN pl. VIII, fig. 2 c gives as the hypostoma of »Ph. caudata» is in fact the hypostoma of a species of Lichas.

_D. Mac Coyi_ BARR. Supl., pl. 13 f. 32. This magnificent hypostoma belongs to the same group as D. micrurus HALL as to which we may hesitate whether there not be a double pair of maculæ above each other near the posterior border. In this species, however, there is the transverse lower groove and above probably the two maculæ united by a groove so closely as to resemble the lower groove.

_D. micrurus_ J. HALL, Pal. N. Y. III pl. 74 fig. 20. Shows, as it were, a double pair of maculæ, very like D. spinifer BARR.

_D. rugosus_ BARR. pl. 24 f. 23. The two maculæ in the ordinary place, without being united by a groove.

_D. socialis_ BARR. pl. 26 f. 21. Rather of an uncommon shape, not so triangular as in the other species, the two maculæ distinct as narrow slits. P. 553 »Vers l'extrémité du corps central, on aperçoit de chaque côté, près du bord, une impression oblique, alongée et arquée».

_D. spinifer_ BARR. Pl. 25, fig. 20. A large beautifully preserved hypostoma, shows what I think we positively must interpret as a double pair of maculæ close above each other. As stated above there is reason to believe that in other species of Dalmanites there also are indications of four maculæ, though not so evident as in this species. BARRANDE says nothing about this remarkable feature.

=Dalmanites imbricatulus= ANGELIN.

Pl. III figs. 43, 44.

The interesting image, as given by the agglomerated eyes, is represented in the magnified figure 43 Plate III. The lenses lie there separated from each other at much varying distance, some in close contact. The presence of the delicate covering membrane is clearly seen, being of white colour contrasting with the black, glossy lens. It is much lacerated and only preserved around the periphery. The size of the ocelli is on an average 0,5 mm., a little one has 0,3 mm. The surface between the ocelli is most finely granulated. The granules scarcely attaining the fourth of the granules in Dalm. vulgaris.

In a vertical section the regular biconvex lenses are seen to be covered with the extremely thin integument, which is a direct continuation from the test. The test between the lenses is perforated by some longitudinal tubes, as usual in the skeleton of the trilobites. We have not found any hypostoma with the maculæ.

=Dalmanites obtusus= LDM.

Pl. III figs. 45, 46.

We have not succeeded in finding any hypostoma, but as the structure of the cephalic eyes is sufficiently well preserved, we here describe it. The lenses seen from the surface look globular, and in a vertical section they are ovate and in a horizontal circular. They are covered by a very thin and delicate membrane, that envelops their superior moiety completely and between the eyes it is enclosed by the surrounding test and growing out from it. In the horizontal section, fig. 45, it looks as a circular frame around the lenses and the membranes of two contiguous lenses sometimes lie so close, that they entirely fill up the interspace between the lenses, leaving only a narrow slit between them marking their boundaries. The space between the lenses, fig. 46, is porous, being perforated by tubes, which continue vertically down through the shell of the cheek.

_Dalm. sclerops_ DALMAN.

Pl. IV fig. 3.

Commonly this species has been ranged with Phacops, but it is evident that it as to the conformation of its glabella and the head in general is highly discrepant, and in these respects is more concordant with Dalmanites, though still deviating as for instance in the pygidium and some details of the head. Therefore it may be justified with FRIEDR. SCHMIDT[41] to consider it as a generic division, though not, as he has it, as an independent genus, but as a subgenus of Dalmanites.

[Footnote 41: Revision, I, p. 76.]

The granulated hypostoma differs from that of Dalmanites proper in having the anterior margin strongly arched, the pointed wings almost on the median line of the hypostoma. There is one groove near the posterior border and the maculæ lie as two, narrow long, crescentic grooves close below the wings. The few sections give no clear idea of any structure.

The cephalic eyes are constructed on the same plan as in the previous species, the lenses being surrounded by a frame.

_Dalm. vulgaris_ SALTER.

Pl. III figs. 47-52.

The hypostoma has the common shield-like shape, fig. 51, anteriorly slightly arcuated, the short obtuse wings curvated towards the interior surface. It is finely granulated. The groove does not go so near the cuspidate posterior margin as in other species. The two maculæ, fig. 52, are narrow and elliptic, somewhat convex and lie in a little concavity, a narrow smooth space around is devoid of the granules, which cover the surface of the rest. They show absolutely no structure, excepting a few indistinct blackish spots.

The cephalic eyes have been often described and delineated. Most detailed are the descriptions and figures given by SALTER in the Memoirs of the Geological Survey Dec. II. pl. 1. In the figure 4 a part of the surface is shown in well preserved state. He says that »the cornea is ... present and distinctly convex over each lens, the intermediate portions being ornamented with tubercles and granules». He thinks also that there are undeveloped or small lenses between the larger. His fig. 5 represents the frames around the lenses, these being probably lost. His explanation of fig. 6 I cannot understand. There are weathered or spoilt lenses figured.

There is no doubt that the granulated surface of the head continues between the more or less free lying lenses as shown here in a new figure taken from a specimen from Dudley, fig. 47. The granules of the surface are larger and more rare between the ocelli than for instance in Dalm. imbricatulus. Small lenses, undeveloped as SALTER calls them, may in fact only be such granules of somewhat larger size than usual. Lenses having the appearance as in SALTER'S fig. 6, as if a covering was partially destroyed and the lens visible below it, I have also found in Swedish specimens, as represented in fig. 47. In a vertical section, fig. 50, a delicate covering integument is seen and the oval lenses lie regularly with their frames around them quite as in Dalmanites obtusus and in horizontal sections, figs. 48, 49, there are also the same sort of black, irregular dots, being the sectioned tubes of the surface between the lenses. But in another feature there is great interest. Beneath each lens there are fascicules of tiny rods, twice as long as the lenses. They are represented in a vertical section in fig. 50 and in a tangential section in fig. 49.

As our imperfect knowledge of so delicate anatomical structures in fossil crustaceans does not admit of secure comparisons with the visual organs of recent crustaceans, no suggestion can be given to interpret their nature. Probably they have no connection with the structure of the eyes and it is not even certain that they are of organic origin.

=Dysplanus= BURM.

If there were no other characters to distinguish this genus from Illænus its strangely deviating hypostoma must do it. We have examined two species and in both the hypostoma is almost oval, evenly rounded both anteriorly and posteriorly. Near the anterior border two narrow, long hornlike wings project. They are flattened, thin and lamellar from the basis and end in a hollow, acuminated point.

=D. centrotus= DALM.

Pl. III figs. 53-56.

This species has the maculæ placed near the posterior border, in a semilunar flat, devoid of the terrace-lines. The maculæ are elongated tubercles and do not show any structure when sectioned. The cephalic eyes have a thin membrane covering the short cupshaped lenses, which, closely packed, in the transverse section give the usual polygonal mosaic of hexaeders. They have a radiate structure, when decayed.

_D. ladogensis_ HOLM.

Pl. III figs. 57, 58.

A specimen from Öland has the maculæ lying in the large field of terrace lines above the smooth zone at the posterior margin of the hypostoma. They are larger than in the former species, pearshaped, smooth and no structure visible.

=Encrinurus= EMMR.

I cannot find any former figure indicating the maculæ of the hypostoma in this genus than the only one of ANGELIN, Pal. Scand. pl. IV fig. 6.

As to the shape of the singularly formed hypostoma in the three Swedish species Encr. punctatus, obtusus and lævis we have found the anterior border so characteristic in them that it is sufficient to distinguish them from each other and to annul the doubts concerning the distinction of Encr. lævis from Encr. punctatus. We therefore in pl. IV figs. 12-17 give the anterior margins of these three species, along with the profiles.

_Encrin. punctatus_ WAHLENBERG.

Pl. IV figs. 5-9, 12, 13.

The surface of the hypostoma is so finely and obscurely granulated that it seems to be almost smooth. It is in the shallow groove above the lamellar tongueshaped posterior border that the elongated tubercular macula are situated. They are prominent and in horizontal sections they have a surface mottled by irregular black and white specks, the whole surrounded by a white border, fig. 6. This border and the other surface is pierced by series of parallel slitlike pores, fig. 7, a feature peculiar to this genus. The vertical sections indicate a chain of indistinct prisms with a black, rounded central hollow between thick strata of clear whitish shell substance, figs. 8, 9. The shell substance on the sides is pierced by the straight, black tubes.

The cephalic eyes have the prisms indistinctly developed or badly preserved with a diameter of 0,04 millim. and height of 0,07 millim. In Encr. lævis the prisms are more distinct and larger with a diameter of 0,00 millim. (pl. IV figs. 10-11).

The maculæ are well developed, ovate, in E. lævis and obtusus and in the same position as in E. punctatus.

=Griffithides= PORTLOCK.

The hypostoma of Griff. globiceps PHILL. shows according to the figure given in H. WOODWARD'S work pl. VI fig. 5 two somewhat obscure maculæ, but no mention is made of them in the description.

=Harpes= GOLDF.

_Harpes d'Orbignyanus_ BARR. NOVÁK II Taf. I fig. 4, with small black maculæ. Harpes venulosus CORDA. NOVÁK II, Taf. I figs. 1, 2. Two narrow, horizontally placed maculæ.

=Harpina= BARR.

_H. prima_ BARR. NOVÁK II Taf. I fig. 5. Two maculæ in continuation of a narrow groove, at the base of the central globosity.

=Holmia= WALCOTT.

_Holmia Lundgreni_ MOBERG, Sveriges äldsta trilobiter, Tafl. 14, fig. 10, 11, 12. The maculæ are much prominent and tubercular, elongate narrow, oblique but not mentioned in the description. Hypostoma probably belonging to the terrace-line group. As well seen in original specimens and in the figures in Dr. HOLM'S paper on Olenellus (Holmia) Kjerulfi there are also in this species two distinct macula; near the posterior margin of the hypostoma, above the marginal groove.

=Homalonotus= KÖNIG.

Maculæ have been previously observed in the following species.

_Hom. delphinocephalus_ GREEN, HALL Pal. N. York vol. II pl. 68 fig. 11, incomplete figure with two well marked tubercular maculæ. No description.

=Hom. Knighti= KÖNIG. SALTER II, pl. 12 fig. 10. Maculæ well expressed as oblique tubercles. On page 120 it is said »with a pair of lateral tubercles well developed (as in Asaphus ...)».

This species is identical with ANGELIN'S Homalon. rhinotropis, Pal. Sc., pl. XX fig. 1 e and his figure has two hollow maculæ seen from the inside of the hypostoma.

=Homalonotus Knighti= KÖNIG (= Homal. rhinotropis ANGELIN).

Pl. IV figs. 20, 21.

The maculæ lie as oblong, smooth tubercles surrounded by the irregularly grown granules of the surface, just below the central globosity of the hypostoma. In a preparation from the interior surface of the macula, as seen in transmitted light, fig. 21, this is white with only a few grey spots and on the superior border black streaks, lying obliquely. The substance of the shell around the macula is perforated by the minutest pores amongst which a few larger are intermingled. The granules are also perforated as those in Calymmene and the tubes continue through the shell, visible by their black colour.

=Hysterolenus= MOBERG.

_Hyst. Törnquisti_ MBG., En trilobit från Dictyograptusskiffern p. 320. In the descriptive letter-press, there is said, that in the exterior (lateral) parts of the anterior groove of the hypostoma a distinct tubercle is to be seen on each side. But none of the figures given, Pl. 17 figs. 6, 7, shows them or at least very indistinctly.

=Illænus= DALMAN.

It is remarkable, considering the great number of species in this almost exclusively Lower Silurian species, that there is none but a single species in which the maculæ hitherto have been delineated. This is Illænus angustifrons var. depresses HOLM I, pl. VIII fig. 18. A little, fragmentary hypostoma with globular maculæ.

=Illænus Chiron= HOLM.

Pl. IV figs. 22-25.

The hypostoma has the shape characteristic of all true Illæni, viz. a straight anterior margin with two large flat rectangular or nearly quadratic wings, a globular or spherical median field, forming the chief portion of the exterior surface, which is covered by some rare, fine, terrace lines. In some other Illæni the surface is smooth. At the posterior base of the median field the two maculæ are placed, tiny, oblong, but lying on the same level, at right angles to the median longitudinal axis of the hypostoma. They are covered with lenses of irregular and indistinct shape fig. 25. I do not think that they in this and the other species of Tiberius are lenses of the same conformation as in Bronteus, they are rather the tops of the subjacent lenses which are of the prismatic form. The cephalic eyes do not show any lenses on the outside. They have a narrow, opaque exterior integument, hiding hexaedral, thickly packed, straight prisms, forming a stratum in thickness of 0,2 millim.

The hypostoma consists of several thin layers of superposed calcite, which easily peal off, so that it is seldom that the true exterior sculptured surface remains.

=Illænus Esmarki= SCHLOTH.

Pl. IV figs. 26-33.

This the most common of all the Illæni has in several specimens shown the hypostoma with the maculæ in situ. The hypostoma is smooth excepting on the marginal ridges where there are some terrace lines. On the median axis of the central gibbosity (fig. 30) five faintly visible transverse folds are situated, a curious feature which we have not observed in any other trilobite. The maculæ are exceedingly small, fig. 31, elongate, spindleshaped and acuminated in both ends. On their surface there are accumulated several lenslike globules. As the horizontal sections show there are, however, no regular lenses, but a network of polygonal meshes with radii in the hollows, fig. 32. The perfect identity of this structure with that of certain states of the cephalic eyes will at once be perceived by comparing this figure with figure 26 representing the same sort of polyedric meshes, though a little more regular in the cephalic eye, where, however, the irregularity also prevails near the periphery of the eye. If we now compare the sections of the cephalic eye of Illænus Chiron and the section of the regular and nearly unchanged eyes of Ill. Esmarki with the second section of the latter, we cannot avoid the conclusion that we in those of Ill. Chiron, figs. 22, 23, and the first of Ill. Esmarki, fig. 27, see the primitive and intact state of the lenses and in the last sections of Ill. Esmarki, figs. 26, 28, the changed and deteriorated state of the same prismatic lenses.

The vertical section of the hypostomic eyes fig. 33 shows in their present changed condition an evident longitudinal direction of the chief elements, quite as the lenses of the eyes. But in how far there ever has existed such prismatic lenses, now changed, we have no evidence definitely to ascertain, but the probability seems to be great that this was the case.

=Illænus gigas= HOLM.

Pl. IV figs. 34-37.

The maculæ in this as well as the other Illæni are so faintly elevated and so inconspicuous that the greatest attention is needed to find them.

The maculæ are elongated, narrow, elliptic on the same level and parallel with the straight anterior margin of the hypostoma. A cast of the left macula is pitted by numerous marks of the lenticular globules of the surface, fig. 37. In the cephalic eye the prisms are seen translucent beneath the cornea. Near the superior border they are larger, fig. 34, almost the double size of those near the inferior border, fig. 35.

=Illænus sphæricus= HOLM.

Pl. IV figs. 42, 43.

Comes near to the previous species. The globules on the surface of the elliptic maculæ are not, however, spread over the entire macula, leaving only a narrow border all around free.

The lenses in the cephalic eyes fig. 42 are globular as seen on the surface, alike those of the maculæ.

=Illænus Roemeri= VOLLBORTH.

Pl. IV figs. 38-41.

The maculæ lie on elevated tubercles and are of a strange shape, being sharpely pointed outwards, rounded inwards. That in fig. 39 is still more peculiar with a narrow stripe on the interior and upper margin. The lenses are larger and more distinct than in any other species of this genus.

Beside the species now mentioned we have also observed hypostomas with maculæ in Illænus laticlavius EICHWALD from Estland, in Illænus Linnarssoni HOLM, and in an unnamed species from Dalecarlia figured without macula by Dr. HOLM in his memoir on the Illænidæ pl. III fig. 22. We have not seen any lenses on the maculæ of these three lenses.

=Lichas= DALMAN.

We have not been able to find any notice about the exceptional maculæ of this genus. There is only a figure of the hypostoma of Lichas hylæus HALL Pal. of N. York vol. VII pl. XXV fig. 5 which would seem to show a pair of narrow, oblique maculæ, but these are quite without resemblance with the maculæ of the true Lichas.

The hypostoma of Lichas is of a characteristic type, broad, with the incised posterior margin and its two lappets reminding of Asaphus, with its large rounded, faintly elevated median part near the anterior margin reminding of Illænus, as also do the lenticles of the maculæ. The maculæ are of a minimal size, scarcely 1 millim. in diameter and thence easily overlooked. Pl. IV figs. 46, 48.

As the hypostomas in this genus generally are found detached it is often difficult to decide to which species they have belonged. So it is with that delineated in Pl. IV fig. 44 natural size. The two maculæ are situated near the centre of the hypostoma, just below the grooves which surround the large, globular disc. They are sunk in a little cavity, rounded and entirely covered by globular lenses (pl. IV fig. 46).

In another detached hypostoma of an unknown species the macula is smaller and the few lenticular globules larger than in the other, fig. 48.

=Lichas latifrons= ANGELIN.

Pl. IV fig. 49.

The maculæ, seen in a thin section from the inside, are reticulate, fig. 49, or of a pattern exactly like that in the changed eyes or maculæ of Illænus and even Asaphus, and thus indicating that it was composed of short prismatic lenses. The vertical section of a macula of a specimen from another locality rather suggests the presence of large sphærical lenses than elongated prisms. The great thinness of the macula in contrast to the excessive thickness of the shell around the eye is seldom so evident as here.

A specimen of an unknown species, found detached, has the macula? unlike the other species, oblique with the inferior end directed inwards, placed on the top of a sort of a little elevation, fig. 47.

=Megalaspides= BRÖGGER.

This author has a figure of M. dalecarlicus in II pl. 1 fig. 19 and also of an unnamed species fig. 20, both showing globular maculæ. In HOLM'S original description of his Megalaspis dalecarlicus (Trilobiten des Phyllograptusschiefers Dalecarliens figs. 8, 9) the same hypostoma is also figured. This genus seems to have a closer affinity with Ptychopyge than with Megalaspis.

=Megalaspis= ANG.

The following figures of hypostomas with maculæ have been published.

_Megal. limbata_ by BRÖGGER I, tab. XII, fig. 10, and in II pl. 2, fig. 22, somewhat differing from the first figure.

_Meg. planilimbata_ BRÖGGER II tab. 2 figs. 21, 21 a. Both figures incomplete and incorrect.

The hypostoma has a large central ovate gibbosity with the maculæ near its basis, surrounded by the winglike expanded limbus.

=Megal. attenuata= WAHLENBERG.

Pl. V figs. 1-6.

The long and narrow maculæ are crescent-shaped and project beyond the hypostoma supported on the superior surface of a sharp extenuated edge where they rest as on a shelf. They consist, as seen in vertical sections, figs. 5, 6, of alternating rows of light and black streaks, and in a horizontal section the spongious network appears, as it is found in Asaphus and others and thus indicating the tendency of this organ to assume a prismatic structure though undeveloped.

The cephalic eyes in M. attenuata form the most regular hexaedral prisms fig. 1 a, short and covered by a thin cornea. The border zone of the eyes has a structure exactly like that of the maculæ, the same as in the spongious zone of Asaphus.

=Megal. limbata= BOECK.

Pl. V fig. 7.

We give a new figure of BRÖGGER'S original specimens, as his figure 23, Tab. II is incomplete and incorrect. The maculæ are situated a little higher up on the sides of the central gibbosity.

=Megal. planilimbata= ANG.

Pl. V fig. 8.

The maculæ are crescent-shaped and not so prominent as in Meg. attenuata and the visual spot in the same position as there.

To observe is that the general structure of the hypostomic shell in this and many other species is prismatic.

=Nileus= DALMAN.

Hypostoma with maculæ figured in N. armadillo by ANGELIN pl. XVI fig. 5 c, and BR. II pl. 3 fig. 40.

The hypostoma in this genus is transversal with large lateral expansions, the central convexity only faintly elevated and almost coherent with the posterior part. The maculæ lie exactly on the horizontal median line of the hypostoma. The anterior wings are well developed like slightly bent acuminated horns and the posterior wings are conical points. The terrace lines are transverse and a little wavy.

=Nileus armadillo= DALM.

Pl. V figs. 10-15.

There is some variability in the hypostoma of specimens from different localities. The most known and numerous specimens are from the renowned localities of Östergötland. They are decidedly transverse, the proportion of the hight to breadth is as 3 to 5 and the narrow border runs all round. In specimens from Kongslena, Vestergötland, the proportion is as 15 to 22, in specimens from Sandvik, Öland, again, we have 13 to 23 or 6 to 11. The maculæ, fig. 14, are oblong, inwardly pointed, quite smooth, with a little concavity in their centre, and their surface is slanting obliquely towards the surface of the hypostoma, almost as in Niobe. There is no trace of any structure.

The cephalic eyes are covered by a comparatively thick membrane, a direct continuation of the general test, fig. 12, hiding a row of prismatic lenses. In some specimens elongate, clear, crystalline rods continue downwards from the bases of the lenses and are probably nothing but inorganic crystals of calcareous spar, fig. 11.--The eyes are larger comparatively than in other species.

=Nileus (Symphysurus) palpebrosus= DALMAN.

Pl. V figs. 16-18.

As to its general form the hypostoma resembles that of the preceding species, but there is no border line, which is so characteristic to the former. The maculæ have the same form and are likewise situated on the median line. The cephalic eyes have prismatic lenses with much convex bases, in a transverse section they are hexaedral and of internal radiate structure.

=Niobe= ANGELIN.

The following figures have formerly been given of its hypostoma with maculæ.

_N. emarginula_ BR. II tab. 2 fig. 33.

_N. explanata_ BR. II tab. 2 fig. 35.

_N. frontalis_ BR. II fig. 37.

_N. insignis_ BR. I tab. IV fig. 1 d, 11 tab. 2 fig. 28.

_N. læviceps_ BR. II pl. 2 fig. 34.

The maculæ are distinguished from all other sorts of maculæ, excepting those of Megalaspis, in being, as it were, shelved on a sort of support, formed through the bulging of the hypostomic surface just below them, one such console for each. Their surface lies consequently almost rectangularly to the adjoining surface of the hypostoma. They are white and smooth.

=Niobe frontalis= ANG.

Pl. V figs. 19-21.

The pocketlike consoles are shorter and broader than in N. læviceps and the maculæ also larger and of an elliptic shape. In a horizontal section near the surface the macula has a speckled appearance of black dots, indicating an undeveloped prismatic structure, much more primitive than in the Asaphidæ.

=Niobe læviceps= ANG.

Pl. V figs. 22-26.

It differs from the preceding through longer consoles below the maculæ, which are oval and in a horizontal section exhibit the same spongious structure, pl. V fig. 25. The cephalic eyes consist of polyedric prisms, which through deterioration show a central cylindrical core surrounded by darkish matter like Asaphus. The passage from solid, homogenous prisms to changed ones is most evident in the figure 22.

=Ogygia= BRONGN.

Figures of the maculæ are found in

_O. Buchi_ SALTER, Mem. Geol. Surv., Dec. II pl. VI fig. 3 p. 2, where it is stated that »there are two transverse furrows near the apex, with compressed tubercles between them» There are two narrow crescentic maculæ and between them a little lower a longer, crescentic ridge, which may be the ridge that in other species connects the maculæ. Moreover, the shape of the hypostoma is not concordant with that of the other species. In Monogr. Brit Trilob. pl. 15 figs. 2, 3 there is only a reproduction of the former figure.

_O. corndensis_ MURCH. SALTER in Monogr. pl. 16 fig. 10 interior side of the hypostoma with two lateral maculæ. In the description two pairs of furrows the uppermost is the maculæ. H. WYATT-EDGELL »On the Genera of Trilobites Asaphus and Ogygia and the Subgenus Ptychopyge» in Geol. Magaz. 1867, p. 14, 15 fig. 2 probably a cast with two oblique impressions of maculæ.

_O. dilatata_ (Asaphus) SARS var. Sarsi ANG. SARS in Isis 1835 p. 342, pl. IX fig. 11. The figure has two small maculæ near the posterior margin and the author says »mit einer starken Vertiefung oder Einschnitt an jeder Seite des Endes». ANGELIN pl. XLII fig. 1 _b_. BRÖGGER'S fig. 38 pl. 3, in II, collies near to the specimen, which is described below, also belonging to Dr. G. HOLM.

=O. dilatata= var. =Sarsi= ANG.

Pl. V figs. 27, 28.

The hypostoma is in so far deviating that the terrace lines have a nearly vertical direction and in this respect much resemble those of the Phillipsiæ and the Proeti. The maculæ, near the exterior angles of the border groove are pear- or spoon-shaped with the apex directed downwards and outwards, and in consequence the longitudinal axis quite opposite to that of the usual direction. The surface is smooth, a little concave. No structure has been discovered. From the pointed apex a slightly elevated ridge runs along the posterior border of the hypostoma and joins the apex of the other macula.

=Paradoxides= BRONGN.

The oldest record of any hypostoma at all amongst the trilobites is that given by WAHLENBERG in his Petref. Suecana (1818) p. 37 Tab. 1, fig. 6, which he, however, considered as the head of the trilobite which he named Entomostracites bucephalus. Figures with maculæ have been given of

_Par. Davidis_ SALTER Mem. Geol. Survey Dec. XI pl. X, fig. 3, a fine figure of a hypostoma with two large oblique maculæ. LINNARSSON in »de undre Paradoxideslagren vid Andrarum» pl. II fig. 2 delineates a gigantic hypostoma with two crescentic tubercular maculæ, having 10 mms. in length.

_Par. Forchhammeri_ ANGELIN Pl. II fig. 3. BRÖGGER, Paradoxidesskiffr. vid Krækling tab. II fig. 10 two marks somewhat different from ANGELIN'S figure. LINNARSSON 1. c. pl. I figs. 9, 10 two hypostomas with maculæ.

_Par. Tessini_, var. _Wahlenbergi_ ANGEL. Pl. I a, fig. 1 b.

_Par. Tessini_, var. _oelandicus_ ANG. Tab. I a, fig. 2 b. ANGELIN'S figure of this hypostoma is incomplete in so far that he had not found the peculiar falciform horns on both sides of the posterior border. Pl. V, fig. 33.

_Par. rugulosus_ CORDA has, according to BRÖGGER, Krækling, pl. II fig. 2 two long, straight callosities, probably corresponding to the maculæ in other Paradoxidæ.

CHR. BOECK gives in Mag. for Naturvidenskaberne Bd 8, 1828, in his paper on the Trilobites on the plate fig. 16 the hypostoma of a Bohemian species with well developed concave maculæ, probably a cast. He compares it with the Entom. bucephalus of WAHLENBERG, but says he cannot explain its nature.

The maculæ of P. oelandicus, pl. V, fig. 34, are tubercles, oblong, smooth, and have invariably an oblong scar along the centre, where probably once a thinner membrane, as in the cephalic eyes of Calymmene, contained the lenses.

=Peltura= M. EDW.

=Peltura scarabæoides= WAHLENB.

Pl. III fig. 42.

We have given a figure of the hypostoma, showing two large, elongated tubercular maculæ or rather resemblances of such, as they are covered with terrace lines as well as the rest of the surface. They have, however, the same position as real maculæ might have, but are directed in a line parallel with the longitudinal axis of the hypostoma. The cephalic eyes have been described above at page 29.

=Phacops= EMMRICH.

Maculæ figured of Phac. (Acaste) Downingiæ. SALTER Monogr. pl. 2 fig. 34 b, p. 25 »high up on each side a small tubercle».

_Phac. rana_ HALL Pal. N. Y. vol. VII, pl. VIII A, fig. 18. Hypostoma with the large commalike maculæ high up.

Of the following species Ph. cephalotes and Ph. macrophthalmus belong to the genus Phacops proper, while Phac. quadrilineata, as well as Ph. Downingiæ are of a quite different generic type. The name Acaste proposed for them is not good as LEACH in 1817 named a Cirrhiped as Acasta.

=Ph. cephalotes= BARR.

Pl. V figs. 35, 36.

The hypostoma is elongate, almost triangular, tricuspidate at the posterior border, a little above this there is a crescent-shaped ridge, and still a little higher above this and the median horizontal line, the two macula, as diminutive tubercles. Seen with magnifying powers they are slightly oblong or reniform.

=Ph. macrophthalmus= BURM.

Pl. VI figs. 1-9.