Chapter III (3)

The triangular hypostoma has a little below the median horizontal diameter two shallow grooves, and a little above the same diameter the extraordinary minute maculæ may with some care be observed, figs. 8, 9. They are visible as a small bare, smooth spot amongst the curious elevated and incoherent terrace lines of the surface. The exceedingly small size of these maculæ naturally does not allow any research of their structure. They are rather in this species and many of the congeneric to be regarded as small ocelli. In some specimens these spots are quite rudimentary and are reduced to the smallest area possible. The anterior wings are of a peculiar earlike shape, finely striate, enlarged and flat near the hypostoma and ending in a little hollow horn (figs. 6, 7). A little below it, the posterior wing emerges as a small angular knob. The aggregate cephalic eyes are more distinct.

As the vertical section learn us the regular ovate lenses consist generally of clear, crystalline calcareous spar and are on the surface covered by an extremely thin membrane, not a common one for all, but a separate cornea for each lens, which envelopes its superior moiety and continues down as a thickened appendix between the lens and the interstitial test (figs. 3-5). In the horizontal sections it surrounds the lenses as an annular wall. Around some lenses there is as if still another ring lay outside this, but more indistinct, probably only clearer portions of the test surrounding them. The cheeks close to the lenses are indistinctly perforated by longitudinal tubes.

=Phacops (Acaste) quadrilineata= ANGELIN.

Pl. V figs. 37, 38.

We have not succeeded in finding any well preserved hypostoma.

The lenses of the cephalic eyes resemble much those of the preceding species though more elongate and ovate. They are covered by a membrane which surrounds their upper moiety and between the lenses joins the interstitial test. In the horizontal section these membranes surround the lenses as an annular wall.

=Phaëtonides= BARR.

=Ph. Stokesi= MURCH.

Pl. VI fig. 11.

The hypostoma somewhat reminds of that of Calymmene, has projecting angles and a prominent knob below the straight anterior border. The two oblique and oblong macula are tubercular and placed near the lateral borders. This hypostoma is upon the whole much related to that of Phillipsia and Proetus.

=Phillipsia= PORTLOCK.

DE KONINCK figured a hypostoma of Phillipsia in his »Description des Animaux Foss. de Belgique» as Cyclus Brongniartianus pl. LII fig. J and NOVÁK in III figs. 6, 7 copied it and corrected the error. It is provided with two globular maculæ united by a curved ridge.

_Phillipsia Eichwaldi_ WOODW. NOVÁK III fig. 5 a hypostoma with globular maculæ. We have been able to examine three different, unnamed or undeterminated species, which we distinguish by numbering them.

_Phill. No. 1_ from the Keokuk group of Crawfordsville, N. America. The hypostoma is broader than in the other species. The nearly horizontal elongated maculæ placed near the median line, close to the lateral margins.

_Phill. No. 2_ (pl. VI, figs. 15-18) probably from the Carboniferous formation of Belgium is of a lengthened form. The elliptic maculæ in the groove, below the central elevation. They are smooth and surrounded by a flat border.

The cephalic eyes are much destroyed, but seem to have consisted of somewhat semi-prismatic lenses or prisms of the same type as Dysplanus.

_Phillipsia No. 3_ (pl. VI figs. 12-14) from the Carboniferous strata of Beeren Eiland communicated by Hr J. G. ANDERSSON. It comes near the preceding, is lengthened, but has broader and shorter anterior wings and the inferior surface just above the posterior border peculiarly pitted. The macula lying above this pitted field are ovate. The lenses of the cephalic eye are hemispheric on the surface.

=Platymetopus= ANGELIN.

=Pl. planifrons= ANGELIN.

Pl. IV figs. 50, 51.

This genus may well be kept distinguished from Lichas in consequence of its peculiar hypostoma and its still more peculiar maculæ, not to remind of the other deviating features. The maculæ lie at the end of narrow, sigmoid grooves of their own above the long transverse groove, separating the posterior border from the anterior elevated moiety. They are elongated, curved and entirely covered with a great number of irregular lenticles, translucent through the thin covering membrane.

=Pliomera= ANG.

_Pliomera Törnquisti_ HOLM in Trilobit. Dalecarliens p. 5 fig. 2.

While Pl. Fischeri and Pl. actinura not have any hypostomic maculæ, this species, which Dr. HOLM with some doubt joins in the same genus, has a hypostoma, which as to its general shape resembles that of both the mentioned species, evenly rounded at its posterior border, with a shallow groove encircling the central field. A little below the horizontal diameter of this the maculæ stretch obliquely inwards and downwards. They rest on a little elevation and are elongate, anteriorly acuminated. The figure on HOLM'S plate does not show the maculæ.

=Proetus= STEININGER.

There are some indistinct indications of maculæ in several of the hypostomas figured in BARRANDE'S Système pl. 15.

_Proet. signatus_ LINDSTR., Gotl. Trilob. Pl. XV fig. 17, shows very distinctly the two maculæ.

=Proet. concinnus= DALM.

Pl. VI figs. 19-23.

In this, as well as in the few other species of Proetus which we have studied, there is that peculiarity of the surface of the hypostoma that two more or less distinct ridges form an acute angle just below the faintly arcuated anterior border. The terrace lines are exactly alike those of the Phillipsiæ, only visible on the central field and ceasing just above the maculæ, where there is a transverse smooth field. The posterior margin has two pair of very short points, one on each side. The macula are lengthened tubercles, somewhat bent and have an oblique position. They are surrounded by tiny prickles and as seen in fig. 23 the macula proper forms a white, elliptic spot on the tubercle. The cephalic eyes are lengthened prisms covered by a thick membrane. They are convex at their base and in a transverse section they show a radiate structure of their interior.

_Proet. conspersus_ ANGELIN.

Pl. VI figs. 24-26.

We have figured a decorticated hypostoma with impressions of two reniform maculæ. The vertical sections of the cephalic eyes show a thin stratum where the lenses are much shorter than in the previous species and covered by a thin film of the common test. The same is also the case in a species which probably is Proet. verrucosus.

_Proet. signatus_ LINDSTRÖM.

Pl. VI figs. 27-30.

On the lateral borders of the hypostoma there are short spines, two on each side, but they are placed differently on the specimens and even wanting in some. The elongated, elliptical maculæ bear in their lower end a white spot on which is seen a little cluster of a varying number of small, segregated ocelli, from three to five, according to the different specimens, like so many black points. The maculæ attain the exceptional length of 4,7 millim. in some specimens, but commonly only 2 millim. We have as yet not been able to detect these remarkable ocelli in any other species of Proetus.

Ptychoparia CORDA.

WALCOTT in »Palæontology of the Eureka District», gives on plate X fig. 21 a representation of a hypostoma which near the posterior margin shows two maculæ as narrow, crescentlike ridges and above them two longer and thicker ridges directed obliquely toward the central axis of the hypostoma.

=Ptychopyge= ANG.

Previous figures of hypostomas with maculæ are the following:

_Pt. aciculata_ BR. II, pl. 1 fig. 13.

_Pt. glabrata_ BR. II, pl. 1 fig. 14.

=Pt. aciculata= ANG.

Pl. VI figs. 40, 41

The broad hypostoma bears the elongated, acuminated maculæ in an oblique direction, contrary to what is seen in most other species. The anterior end slopes inwards and the lower end outwards, the surface, which in the cast drawn is surrounded by a marginal line, is directed sidewards against the broad lateral wings and is obscured by the declivity formed by the groove. The same disposition of the mature also occurs in Ptych. glabrata. These two species consequently in this respect differ essentially from the other four species which we have examined, but there is for the rest no reason to make these form different genera or subgenera.

=Pt. angustifrons= DALM.

Pl. VI figs. 31-36.

There are two varieties of hypostomas: one broad fig. 32 and one more elongated fig. 33. The maculæ having a convex surface lie at the inferior end of the lateral grooves prominent on a semicylindrical support, slightly inclining outwards. In the broad variety they are oblique (fig. 34) and in the elongated variety they are horizontal (fig. 35). In a vertical section fig. 36 we have the same indications of a prismatic arrangement in the maculæ as in the Asaphi. The horizontal section does not represent so clear and distinct a reticulate structure as in the Asaphi, the prisms are much smaller.

The cephalic eyes are composed of closely packed hexaedral prisms, the gradual change from solid homogenous ones to those which are interiorly destroyed, only with the exterior form preserved is easily seen. The figure of a horizontal section (fig. 31) gives a good idea of the peculiar destruction of the interior of these prisms. In the vertical section the prisms are not discernible.

=Pt. cincta= BRÖGGER? according to FRIEDR. SCHMIDT in Mus. Holm.

Pl. VI figs. 37-39.

The hypostoma which BRÖGGER figures (II, pl. I fig. 2 _a_) as belonging to this species is completely identical with that of Pt. angustifrons, while the specimens which FR. SCHMIDT has marked out as being the true Pt. cincta have a different hypostoma. The maculæ vary in the specimens, being in some placed horizontally or in a right angle to the longitudinal axis of the hypostoma, in others, again, a little obliquely outwards. They form prominent, free lying tubercles. It is from the superior face that the rudimentary lenses have left their traces, as seen in a longitudinal section (fig. 39). It is consequently this superior surface which is the macula proper.

=Pt. glabrata= ANG.

Pl. VI fig. 42.

We have given a figure of the elliptic acuminate maculæ as seen on a cast of the inside of a hypostoma, the same which is the original to fig. 14 in BR. II.

=Schmidtia= MARCOU.

MOBERG has in his paper on »Sveriges äldsta Trilobiter», pl. 15, fig. 6, drawn a hypostoma of Schmidtia Torelli with scars of maculæ, placed like that of the Paradoxidæ. There is no mention made of them in the descriptions.

=Trochurus= BEYRICH.

=Tr. pusillus= ANG.

Pl. IV figs. 52, 53.

The strange transverse, bipartite hypostoma carries the small, round, tuberculose maculæ on the superior moiety. As seen from the interior (fig. 53) they are obscurely maculate, but it is doubtful whether these black dots are derived from lenses, as the surrounding hypostomic surface also has the same mottled aspect.

=Trochurus= sp.

Pl. IV figs. 54, 55.

This hypostoma belongs to an unknown species of Trochurus from the Southern Gotland (Burs). Its macula are placed almost as in the preceding species, but are much larger. They are surrounded by prominent margins. No lenses have been discovered on them.

CONCLUSIONS.

In reviewing the genera and species enumerated above, as to their number in which the structure of the macula has been observed, we arrive at the following results.

---+----------------+-----------+---------------------+
| | Number of | Number of species in|
| Genera. | species | the maculæ of which|
| | with | the structure has |
| | maculæ. | been examined. |
---+----------------+-----------+---------------------+
1 | Acaste | 1 | -- |
2 | Acidaspis | 1 ? | -- |
3 | Asaphus | 18 | 7 |
4 | Barrandia | 1 | -- |
5 | Bronteus | 9 | 5 |
6 | Bumastus | 7 | 1 |
7 | Calymmene | 4 | 2 |
8 | Centropleura | 1 | -- |
9 | Chasmops | 1 | -- |
10 | Chirurus | 9 | 2 |
11 | Ctenopyge? | 1 | -- |
12 | Dalmanites | 10 | 2 |
13 | Dysplanus | 2 | 2 |
14 | Encrinurus | 3 | 1 |
15 | Griffithides | 1 | -- |
16 | Herpes | 2 | -- |
17 | Harpina | 1 | -- |
18 | Holmia | 2 | -- |
19 | Homalonotus | 2 | 1 |
20 | Hysterolenus | 1 | -- |
21 | Illænus | 9 | 5 |
22 | Lichas | 4 | 1 |
23 | Megalaspides | 2 | -- |
24 | Megalaspis | 3 | 1 |
25 | Nileus | 2 | 2 |
26 | Niobe | 5 | 2 |
27 | Ogygia | 3 | -- |
28 | Paradoxides[42]| 8 | -- |
29 | Peltura | 1 | -- |
30 | Phacops | 3 | -- |
31 | Phaëtonides | 1 | -- |
32 | Phillipsia | 5 | -- |
33 | Platymetopus | 1 | -- |
34 | Pliomera | 1 | -- |
35 | Proetus | 3 | -- |
36 | Ptychoparia | 1 | -- |
37 | Ptychopyge | 4 | 2 |
38 | Schmidtia | 1 | -- |
39 | Trochurus | 2 | -- |
| +===========+=====================+
| Total | 136 | 36 |

[Footnote 42: To these have here been added two Bohemian species P. bohemicus and P. spinosus omitted above at page 64.]

We have thus 136 species of 39 genera in which hypostomic maculæ have been found and only 36 species, in which it has been possible to study the structure of the maculæ through sections.

Common for a great number of maculæ in various groups, whether they show any organic structure or not, is the excessive thinness of their shell in comparison with that of the surrounding hypostoma. This is also in accordance with the tenuity of the cephalic eyes in relation to the test of the cheeks.

The structure which characterizes the macula as a visual organ, although often so rudimentary, is not in all instances spread through the whole substance of the macula. This structure has in several species been restricted to a narrow circumference of the body of the macula, to its interior apex as in Bronteus and Proetus. Only in the Asaphidæ, in Illænus and Lichas the entire macula shows this structure. Perhaps, to judge by certain indications in Bronteus, once in a larval or preceding stage of evolution the whole surface of the macula was also in that genus covered with lenses, which have been reduced.

The different groups in which the examined genera may be divided are the following seven, showing the remarkably great diversity of these organs. There is even in the same genus so great a variability that species with structure in the macula occur along with those devoid of any structure or also, as in Lichas, with a different structure. It must, however, be remembered, that the species of such genera may not be coeval.

_Group 1._ In the sectioned macula there is no trace whatever of any structure. The test of the macula considerably thinner than that of the hypostoma. According to the affinities of the genera they may be subdivided as follows. _a_ Bumastus, Dysplanus. _b_ Nileus, Symphysurus, ? Ogygia. _c_ Calymmene, Homalonotus. _d_ Chirurus pro parte.

_Group 2._ The whole macula is of a spongious or irregularly polyedric structure, showing prisms in vertical sections. Its concordance with the structure of the deteriorated cephalic eyes or with the so called border zone is complete. If a supposition may be hazarded, I think that the spongious or reticulate structure in the maculæ is their real and original state, a lower stage of development of the visual organs, out of which the prisms in the cephalic eyes have been formed, and that the prismatic lenses in their decay, as seen in the Asaphidæ, reveal to us their original state and structure, and thus, as it were, return to the primary stage in the maculæ.

Asaphus, Isotelus, Megalaspis, Ptychopyge, Niobe, ? Megalaspides, ? Barrandia.

Illænus, Lichas, Trochurus, ? Platymetopus.

Encrinurus also probably belongs to this group.

_Group 3._ Maculæ with well developed globular lenses on the interior third of the macula. The blank part of the macula without any structure. In one instance (Chirurus spinulosus) the whole macula covered with lenses.

Bronteus, Chirurus spinulosus.

_Group 4._ The macula form sunk pits with smooth bottom. Structure unknown. In a few species there seems to be no less than four maculæ in two pair above each other.

Dalmanites.

_Group 6._ The elongate straight maculæ carrying on their innermost point from three to five diminutive, segregate ocelli situated on a clear, white patch. Proetus.

As yet these ocelli have not been observed on the related genera Cyphaspis, Phaëtonides, Phillipsia and Griffithides, but they may be preliminarily ranked here in consequence of their close conformity.

_Group 7._ The maculæ have in this group been reduced to a pair of tiny ocelli situated high up on the hypostoma, near its anterior margin.

Phacops, Acaste.

The position of the hypostomic eyes on the ventral surface of the trilobites is not quite so abnormous, nor so isolated a feature amongst the crustaceans as might at first be supposed. Amongst the recent crustacea there is probably none, which as to the conformation of its labrum or hypostoma resembles the trilobites so closely as the species of the genus Apus, however different they may else be in all other respects. We have studied chiefly the hypostoma of Apus cancriformis. In its shape as seen from the outside and as to its outlines it reminds strongly of the trilobite hypostoma. It is nearly square (pl. VI figs. 46, 47) with rounded corners. The anterior margin is arched and in the middle it has a broad tonguelike prolongation. On the sides of the anterior margin there are two short, sharply pointed wings. The lateral margins are thick, almost as doubled, and bent forward so that a groove is formed alongside them and the large, evenly vaulted median surface of the hypostoma. These grooves are continued in a short groove parallel with the posterior margin. From this margin a narrow oblique surface is slanting backwards, as it were, doubling that margin. On its surface there are two small, resplendent, white or clear spots, almost of the shape of a crescent, though more irregular when seen in higher enlargement (pl. VI fig. 49). They are translucent as to be visible on the interior side of the hypostoma. In Apus glacialis there are also similar spots in the same position, but they form short narrow, straight stripes, parallel with the border of the posterior margin. I have no opinion as to the nature or function of these white spots. I do not think that they at all are homologous with the macula; of the trilobites. On the other hand we see a little higher up on the exterior side of the hypostoma two faintly elevated elliptic tubercles which as to their position and shape highly remind of the maculæ of the trilobites. They are hollow on the inside and we have not been able to find any peculiar structure in them. That they have any visual function is precluded through the circumstance that the whole exterior surface is densely covered with microscopic bristles (pl. VI fig. 49) which are especially remarkable along the posterior margin.

In the class of the Cirrhipedia a sufficient number of facts is known to ascertain us of the presence of eye spots on the hypostomas of the larva. The researches of SPENCE BATE[43] show in the larva of Balanus balanoides, on the hypostoma or labrum, which he calls proboscis, an eye, a little from the posterior margin of the labrum. It seems during the various moults of the animal to have changed its place, being in the two first moults situated near the anterior margin of the hypostoma. In Clitia Strömi it lies at the anterior part of the labrum (figs. 9, 10). SPENCE BATE does not accept the opinion of their nature as eyes, but the following authors as DARWIN and CLAPARÈDE agree in considering these spots as eyes. DARWIN[44] describes two such eyes in Lepas australis and, if I catch his meaning rightly, one on each side of the labrum. CLAPARÈDE has given a figure of the larva of Lepas anatifera[45] in the hypostoma of which (by him called »Rüssell», proboscis) a single eye is seen, having at the sides two round problematic organs, that he at first considered as auditory cells, but later as eyes. HESSE[46] has also in a species of Scalpellum seen several coloured spots on the inferior surface of the carapace, below the antennæ and he expressly says that they are eyes and a special pair of supplemental eyes that are becoming functional after the first moult.

[Footnote 43: On the development of the Cirripedia, Ann. Mag. Nat. list. 1851, p. 324, pl. VI figs. 1, 2, 3, pl. VII fig. 10.]

[Footnote 44: Monogr. of Cirripedia I p. 17.]

[Footnote 45: Beobachtungen ... über wirbellose Thiere, 1863, pl. XVII fig. 22.]

[Footnote 46: Mémoire sur les métamorphoses que subissent les Anatifes appelés Scalpels obliques. Annales des Sciences nat. 1859. Tome XI p. 163.]

These sternal eyes in the larva are previous to the eyes in the dorsal side of the later stages and whenever such are discovered in other groups of adult animals as the trilobites they must of course be considered as remnants from the larval stage. As a rule, in several tribes of the crustacea it has been observed that the eyes originate on the ventral or sternal side of the body and in the advancement of growth move to the superior side. So according to PACKARD and others the embryo of Limulus has originally the median eyes on the ventral side. In the same way the cephalic eyes of Dalmanites move, as BARRANDE has shown, from the ventral side to their permanent place on the superior side of the head shield. But while in the Cirrhipedia the larval or hypostomic eyes are lost in the adult they remain on the hypostoma of a great number of the adult trilobites and have probably left their mark behind in A pus. No wonder then that we find these maculæ developed as precursors of the hypostomic eyes in a number of the oldest and in other respects blind trilobites as the Olenellidæ and Paradoxidæ to which they may have been of some use as a sort of visual organs, though probably very imperfect.

We find the maculæ of the trilobites present from the oldest Cambrian times and we find also in them a progressive evolution, in some to a high degree, lenses and facets, perfectly identical with those of the eyes on the head shield, converting them into true eyes. It was in the Lower Silurian times, in the Lower grey Orthoceratite limestone of Sweden, that Illænus Esmarki appeared provided with actual prismatic lenses on its maculæ and probably also Lichas. Still higher at the top of the Lower Silurians, in the Leptæna limestone of Dalecarlia the genus Bronteus has appeared with those highly developed hypostomic eyes.

But there are, no doubt, still more facts to adduce for filling up extant lacunæ in the knowledge of these matters. The material now at hand has, however, not been sufficient for doing this. At present I must leave this interesting field of investigation and be contented if I have succeeded in showing where rich harvests await every one who will make a search for them.

Explanation of the plates.

All specimens figured belong to the Swedish State Museum,
unless otherwise stated.

Pl. I.

=Acidaspis crenata= EMMR. p. 85.

Fig. 1 & 2. Vertical sections of the eye 60/1.
» 3. The outside of the hypostoma 6/1.
» 4. The anterior margin of the hypostoma, showing
one of the clubshaped spots in transmitted
light 20/1.
» 5. The granulations of the hypostoma in transmitted
light 80/1.
» 6. Vertical section of the hypostoma with granules 60/1.
Specimens from Djupvik, Eksta.

=Agnostus glandiformis= ANGELIN p. 35.

Fig. 7. Vertical section of a part of a rolled up specimen 8/1.
Specimen signed CQ. Andrarum, Scania.

=Asaphus expansus= L. p. 39.

Fig. 8. Horizontal section of the eye near the upper margin.
_a._ The surrounding spongious zone,
_b._ The surface of the eye with lenses 60/1.
Spec. CG.
» 9. Horizontal section from the centrum of the eye 60/1. Spec. CF.
» 10. Horizontal section near the inferior margin,
_a_ the eye, _b_ marginal zone, _c_ a portion
of the cheek 60/1. Specim. CF.
» 11. Vertical section near the inferior margin 60/1,
same specimen as in fig. 10.
» 12. Eye fractured vertically showing regular prisms
in the eye (_a_) and irregular ones in the marginal
zone (_b_) 30/1.
» 13. Part of the hypostoma with the macula 12/1.
» 14. The right macula from the inside 12/1.
» 15. A vertical section across the hypostoma,
_aa_ the maculæ 8/1.
» 16. A similar one of the macula at left in the
preceding figure 60/1.
» 17. The right macula from the outside in transmitted
light 60/1. Sp. BQ.

=Asaphus fallax= DALM.? p. 41.

Fig. 18. Horizontal section of the eye 60/1. Spec. CD.
» 19. Vertical section of the same passing obliquely
through two rows of prisms 60/1.
» 20. Fragment of the same eye with entire prisms 30/1.
» 21. The hypostoma seen from the inside 3/1 from
Sandvik, Öland.
» 22. The left macula 12/1. Spec. BO. From Vestanå, Östergötland.

=Asaphus raniceps= DALM. p. 40.

Fig. 23. The left macula 12/1. Spec. BV.
» 24. Horizontal section of right macula 60/1. Spec. BY.
» 25. Vertical section of a macula 60/1.
» 26. A similar section of a macula (_a_) in the
longitudinal axis of the hypostoma 12/1.
Exterior surface at right.
All from Östergötland.

=Asaphus= sp. p. 41.

Fig. 27. A portion of the eye near the superior margin 30/1.
» 28. Horizontal section of the eye 60/1.
» 29. Horizontal section near the inferior margin 60/1.
» 30. Vertical section near the superior margin 60/1.
All figures from specimen CE. From Gotska Sandön.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. I.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. II.

=Asaphus= (Isotelus) =gigas= J. HALL p. 42.

Fig. 1. The right macula surrounded by terrace lines 3/1.
» 2. Horizontal section of a macula with a part of the test 60/1.
» 3. Vertical section of the same, the surface of
the outside upwards 60/1. All specimens from
Cincinnati, Ohio, Hudson River group. Spec. BU.

=Bronteus irradians= LINDSTRÖM. p. 43.

Fig. 4. The hypostoma 4/1.
» 5. The left macula 12/1. Specimen from Gotland, Löjsta.

=Bronteus laticauda= ANG. p. 44.

Fig. 6. Horizontal section of the cephalic eye 20/1.
» 7. The same highly magnified 250/1.
» 8 & 9. Vertical sections of the cephalic eyes 20/1.
In fig. 8 with integument, in fig. 9 decorticated.
» 10. Hypostoma. Specim. N. 2/1.
» 11. The right macula, from the outside. Spec. O. 12/1.
» 12 & 13. Left maculæ, casts of the interior surface 12/1.
Specimens from Dalecarlia.

_Bronteus platyactin_ ANG. p. 43.

Fig. 14. Vertical section of cephalic eye, _a_ the eye.
_b_, cheek 20/1.
» 15. Hypostoma 2/1.
» 16. The same from the interior surface. Sockets
of the maculæ sunk in the shell 2/1.
» 17. The right macula 12/1.
» 18. Portion of the eye on the macula in transmitted light 20/1.
» 19. The same more magnified 60/1. Specimens
from the stratum _b_. Wisby, Norderstrand.

=Bronteus polyactin= ANG. p. 8.

Fig. 20. Horizontal section of cephalic eye 20/1.
» 21. Vertical section of the same 60/1.
» 22. Hypostoma 4/1.
» 23-25. Samples of right maculæ 20/1.
» 26. Horizontal section of macula, the portion
bearing the lenses 20/1.
» 27. Vertical section of the same 60/1, _a_ the
lenses. Specimen from Stora Carlsö.
» 28. Hypostoma 4/1.
» 29. The right macula 12/1.
» 30. A portion of the lenses on the macula in reflected
light 30/1. The lenses are only faintly convex and
encased in a white frame. The diameter of the largest
lenses 0,06 millim. Specimen from Lansa, Fårö.

=Bumastus barriensis= MURCH. p. 46.

Fig. 31. Hypostoma 4/1.
» 32. The right macula 12/1. Specim. CU from Wisby.

=Bumastus sulcatus= LINDST. p. 46.

Fig. 33. Horizontal Section of a cephalic eye 20/1.
» 34. The same magnified 60/1.
» 35. A similar, near the inferior margin and close
to the surface, _a_ the eye, _b_ cheek 60/1.
Compare fig. 38.
» 36. Vertical section of the cephalic eye near the
suture 20/1.
» 37. The same, more magnified 60/1.
» 38. A similar, near the inferior margin, _a_ eye,
_b_ cheek 60/1.
» 39. Cast of the interior surface of a hypostoma 2/1.
» 40. The same seen from the side.
Originals to figs. 33-38 from Samsugu,
Othem, Gotland, figs. 39 & 40 from Stora
Vede, Follingbo, Gotland.

=Bumastus= sp. p. 47.

Fig. 41. Horizontal section of cephalic eye, near the inferior margin,
_a_ eye, _b_ cheek, 60/1.
» 42. Vertical section of the same 60/1.
» 43. A similar near the inferior margin, _a_ eye,
_b_ cheek, 60/1.
» 44. Hypostoma 4/1.
» 45. Side view of the same.
» 46. The right macula 12/1. Specimens from Korpklint near Wisby.

=Bumastus= sp. p. 46.

Fig. 47. Hypostoma 4/1.
» 48. The right macula 12/1. Specimen from Klints, Othem, Gotland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. II.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. III.

=Bumastus= n. sp. p. 47.

Fig. 1. Hypostoma 4/1. Specimen AY. From Wisby, Norderstrand.
» 2. Its left macula 12/1.

=Bumastus= n. sp. (different from the preceding) p. 47.

Fig. 3. Hypostoma 4/1. Spec. AT from Wisby, Norderstrand.
» 4. Vertical section of the macula _a_ in its longest axis.
» 5. A portion of the cephalic eye, the facets very low.

=Calymmene intermedia= LINDSTR. p. 48.

Fig. 6. Hypostoma 4/1. Spec. BG from Gotland.
» 7. The left macula 12/1.

=Calymmene tuberculata= BRÜNNICH p. 48

Fig. 8. The inside of the hypostoma 3/1. Spec. BA from Gotland.
» 9. Horizontal section near the surface, seen from the
inside 20/1.

=Chasmops macroura= (?) p. 49.

Fig. 10. The left macula, cast of the inside 12/1. Rostock.

=Chirurus glaber= ANG. p. 50.

Fig. 11. A portion of the surface of the cephalic eye 12/1.
From Dalecarlia.

=Chirurus ornatus= DALM. p. 50.

Fig. 12. Horizontal section of a Cephalic eye near the surface 60/1.
» 13. Vertical section of the same, 20/1, at the inferior
margin. A line added to show the supposed size of the
fragmentary lenses.
» 14. The left macula of the hypostoma 12/1.
From Östergötland, Westanå.

=Chirurus spinulosus= NIECZKOWSKI p. 51.

Fig. 15. The right macula of the hypostoma from the
inside 12/1. Belongs to Dr. G. HOLM, from
Esthonia, Kuckers C^2.

=Chirurus= sp. p. 51.

Fig. 16. The left hypostomic macula 12/1. Öland, Brunsby kanal.

=Cyrtometopus clavifrons= DALM. p. 49.

Fig. 17 & 18. Horizontal sections of cephalic eyes 60/1.
» 19. Vertical sections of cephalic eyes and a part
of the cheek 20/1.
» 20. A highly magnified portion of the eye in vertical section
60/1.
» 21. Hypostoma, 3/1 the anterior wings completed after another
specimen. From Östergötland.

=Cyphaspis elegantula= ANGELIN p. 52.

Fig. 22 & 23. Horizontal and Vertical sections of cephalic eyes 60/1.
Diameter of facets 0,03 mm.
» 24 & 25. Hypostomas from the front side and
in side view 8/1. Upper Silurian, Gotland.

=Ctenopyge pecten= SALTER p. 29.

Fig. 26. The head shield, the free cheeks added from other
specimens 4/1.
» 27. A free cheek, viewed without foreshortening, as in its
natural position 4/1. Alumschists of Andrarum, Scania

=Ctenopyge= n. sp. (LINNARSSON, Geol. För. Förhandl. 1880, pl. VI, figs. 14, 15) p. 29.

Fig. 28. A free cheek 6/1.
» 29. Surface of the eyes, near the suture 30/1.
» 30. The same next the free cheek 30/1. Andrarum.

=Sphærophthalmus alatus= ANG. p. 29.

Fig. 31. The head shield; the free cheeks have been added 8/1.
» 32. A free cheek without foreshortening 8/1.
» 33 & 34. Horizontal and Vertical sections of cephalic eyes
100/1. The cornea nearly 0,01 mm. in thickness. Andrarum.

=Peltura scarabæoides= WAHLENB. p. 29.

Fig. 35. The head shield; the free cheeks added 2/1.
» 36. A free cheek, not foreshortened 4/1.
» 37. A cast of the inside of the cephalic eye 20/1.
» 38. A portion of the cephalic eye from the inside 60/1.
» 39. A portion of the same in transmitted light 100/1.
» 40. Vertical section of an eye 100/1. The rock on the inside.
» 41. A similar one passing through two rows of lenses, the one
behind the other. The rock on the outside 100/1.
» 42. A hypostoma 8/1. Andrarum.

=Dalmanites imbricatulus= ANGELIN p. 53.

Fig. 43. The cephalic eye near the inferior margin 20/1.
» 44. Vertical section of the same and a portion of the free
cheek 60/1. The lenses are almost contiguous and the
interspace between them is formed by the common test of
the body, which is perforated by longitudinal tubes.

=Dalmanites obtusus= LINDSTR. p. 53.

Fig. 45. Horizontal section of cephalic eye near the inferior
margin 30/1.
» 46. Vertical section, the interspaces being in this wider than
in the preceding 60/1. From Gotland.

=Dalmanites vulgaris= SALTER P. 54.

Fig. 47. Part of the cephalic eye near the inferior margin 20/1.
» 48. Horizontal section near the surface at the inferior
margin 30/1.
» 49. A tangential section, so that the inferior part is below
the lenses where the peculiar radiating structure is seen
in horizontal section 30/1.
» 50. Vertical section of a small Specimen, fibrous appendices
below the lenses 30/1.
» 51. Hypostoma 2/1.
» 52. The right macula 12/1. English specimens from Dudley,
Shropshire.

=Dysplanus centrotus= DALMAN p. 55.

Fig. 53 & 54. Horizontal and vertical sections of an eye 60/1.
» 55. Hypostoma 3/1.
» 56. The anterior wing of the hypostoma seen in foreshortening
6/1. Specimen AX from Westanå, Östergötland.

=Dysplanus ladogensis= HOLM p. 55.

Fig. 57. Hypostoma 3/1.
» 58. The same, seen from the side 3/1. Specimen AI from
Östergötland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. III.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. IV.

=Chirurus speciosus= HIS.

Fig. 1. The right macula, cast of the interior side 12/1.
Specimen from Grumpevik, Vamlingbo, Gotland.

=Cybele bellatula= DALM. p. 52.

Fig. 2. The hypostoma 3/1. Vestanå, Östergötland.

=Dalmanites sclerops= DALMAN p. 54.

Fig. 3. The hypostoma 2/1.

=Encrinurus punctatus= WAHLENB. p. 56.

Fig. 4. Horizontal Section of an eye 69/1. Diameter
of the prisms 0,04 mill., their length 0,07 millim.
» 5. The hypostoma 3/1.
» 6. The left macula. Reflected light 12/1.
» 7. Part of the left macula in transmitted light 20/1.
The black streaks are sectioned pores. Spec. BH.
» 8. Vertical section of the right macula, a sagittal
section 20/1. BH.
» 9. A similar one _a_ along the shorter diameter 20/1.
Specimens 5, 6 from Vikers Boge, 4, 9 Medebys, Hall,
7, 8 Svarfvarehuk. All from Gotland.

=Encrinurus lævis= ANG. p. 56.

Fig. 10. Horizontal section of the cephalic eye.
» 11. Vertical section of the same. Both 60/1.
Prisms 0,05 in breadth, 0,06 in length. Specimen
from Wisby.
» 12-17. Longitudinal profiles and anterior margins of the
hypostomas of Encr. punctatus (figs. 12, 13),
E. obtusus (figs. 14, 15) and E. lævis (figs. 16, 17).

=Harpes vittatus= BARR. p. 31.

Fig. 18. The two ocelli at right 8/1.
» 19. Sagittal Section of two ocelli 60/1. Specimen from Lochkow
E, e^2, Bohemia.

=Homalonotus Knighti= KÖNIG p. 57.

Fig. 20. The hypostoma 3/2.
» 21. The right macula, from the interior side in transmitted
light 20/1. From Scania.

=Illænus chiron= HOLM p. 58.

Fig. 22. Horizontal Section of an eye.
» 23. Vertical section of the same, both 60/1. Spec. AG.
» 24. Hypostoma 2/1. Spec. AF.
» 25. The right macula seen in reflected light. The lenses
translucent 12/1. Ex. AA. All from Persnäs, Öland.

=Illænus Esmarki= SCHLOTH. p. 511.

Fig. 26 & 27. Horizontal sections of the eyes in two different
specimens 60/1.
» 28. Vertical section of an eye 60/1.
» 29. Likewise, _a_ the eye, _b_ a portion of the
cheek 12/1.
» 30. Hypostoma 2/1.
» 31. The inside of the right macula 12/1. Spec. AR.
» 32. The outside of the left macula in transmitted light 60/1.
» 33. Sagittal section of the macula (_a_) 60/1.
All specimens from Vestanå, Östergötland, excepting
fig. 30 belonging to Dr. HOLM, from
Norway, Bjerkaas.

=Illænus gigas= HOLM P. 59.

Fig. 34 & 35. Surface views of the prisms of the eyes, fig. 34 at
the superior margin and 35 at the inferior one 12/1.
» 36. Cast of the inside of a hypostoma 1/1. Same original as
HOLM'S fig. 6 Tafl. I, Illænider.
» 37. Cast of the inside of the left macula. Same specimen 12/1.

=Illænus Roemeri= VOLLBORTH (Ill. vivax HOLM) p. 59.

Fig. 38. Hypostoma 1/1. Spec. AO.
» 39. The right macula; the lenses translucent in reflected light
12/1. Spec. AO.
» 40, 41. Both maculæ of another, larger specimen 12/1. Sp. AP.
From the Leptæna limestone of Dalecarlia.

=Illænus sphæricus= HOLM p. 59.

Fig. 42. The prisms of cephalic eye 12/1. }
» 43. The right maculæ, transparent in }
transmitted light 12/1. Sollerön, } Spec. AH.
Dalecarlia. }

=Lichas= sp. p. 60.

Fig. 44. The hypostoma 1/1.
» 45. Cast of the inside of the right macula 12/1. Spec. T.
» 46. Cast in wax of the preceding 12/1.
» 47. Cast of the inside of the hypostoma. The central portion
with the maculæ 3/1. Spec. V.
» 48. Inside of the right macula 12/1. Spec. Y.
From a quarry north of the gallows near Wisby.

=Lichas latifrons= ANG. p. 60.

Fig. 49. The right macula, transmitted light, seen from the inside
60/1. From Kyrkberget, Wisby.

=Platymetopus planifrons= ANG. p. 67.

Fig. 50. The hypostoma 1/1.
» 51. The left macula. Transparent lenses? 12/1.
From the Leptæna limestone of Dalecarlia.

=Trochurus pusillus= ANG. p. 70.

Fig. 52. The hypostoma 5/1.
» 53. The right macula in transmitted light. Seen from the inside
60/1. From Fröjel.

=Trochurus?= p. 70.

Fig. 54. The hypostoma 3/1.
» 55. The right macula from the inside 12/1. From När Gotland.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. IV.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. V.

=Megalaspis attenuata= WAHLENB.

Fig. 1. Horizontal section of cephalic eye near the inferior margin
60/1. _a_ surface of the eye, _b_ frame with irregular or
reserve prisms, _c_ part of the cheek with normal
structure. Spec. DP.
» 2. Hypostoma 2/1. Spec. BM.
» 3. The right macula 12/1.
» 4. Part of the same in transmitted light 60/1. Spec. DR.
» 5. Vertical (sagittal) section of the inferior part of the
hypostoma, crossing the macula _a_ 12/1.
» 6. Vertical section of the same macula 60/1. All spec. from
Westanå, Östergötland.

=Megalaspis limbata= BOECK p. 61.

Fig. 7. Hypostoma, mostly decorticated 2/1. Specimen from Fogelsång,
Scania, the same as BRÖGGER'S original to his
fig. 23.

=Megalaspis planilimbata= ANGEL. p. 61.

Fig. 8. Hypostoma, Öland, Äleklinta 2/1.

=Megalaspis= sp.

Fig. 9. Vertical section of macula (_a_) 12/1. Öland, Lower
grey Orthoc. limestone.

=Nileus armadillo= DALM. p. 62.

Fig. 10. Horizontal section of cephalic eye 60/1.
» 11. Vertical section of the same 60/1.
» 12. A similar section 60/1. _a_ the prisms of clear
calcareous spar, _b_ the surface of the eye and
the free cheek of yellow spar.
» 13. The hypostoma 2/1.
» 14. The right macula 12/1.
» 15. Vertical section of macula (_a_) 12/1. All specimens
from Persnäs, Öland, excepting fig. 13 from Westanå,
Östergötland.

=Symphysurus palpebrosus= DALMAN p. 62.

Fig. 16. Horizontal section of cephalic eye 60/1.
» 17. Vertical section of the same 60/1.
» 18. Hypostoma 2/1. From Westanå, Östergötland.

=Niobe frontalis= ANG. p. 63.

Fig. 19. The hypostoma 2/1.
» 20. The right macula 10/1.
» 21. Part of the same in transmitted light 60/1.
Specimen from Kungs Norrby, Östergötland.

=Niobe læviceps= ANG. p. 63.

Fig. 22. Horizontal section of cephalic eye near the inferior margin,
_a_ cheek 60/1.
» 23. Hypostoma 2/1.
» 24. The right macula 10/1.
» 23. Part of the same in transmitted light 60/1.
» 26. Vertical section of macula _a_ 12/1.

=Ogygia dilatata= var. =Sarsi= ANGEL. p. 63.

Fig. 27. Hypostoma 2/1.
» 28. The right macula 10/1. From Christiania, Haakavik, Collection
of Dr. G. HOLM.

=Olenus= sp. p. 33.

Fig. 29. A part of the free cheek with the scallop 6/1.
» 30. Profile of the scallop. Andrarum, Scania.

=Parabolina spinulosa= WAHLENB. p. 33.

Fig. 31. The free cheek with the scallop 3/1.
» 32. Profile of the scallop. From Jemtland.

=Paradoxides oelandicus= ANGEL. p. 64.

Fig. 33. Hypostoma 2/1.
» 34. The left macula 12/1. Öland, Torp.

=Phacops cephalotes= BARR. p. 65.

Fig. 35. The hypostoma; the shell mostly decorticated and the
maculæ visible as casts on the inside.
» 36. Cast of the inside of the right macula 12/1. Spec. from
Lochkow, Bohemia.

=Phacops quadrilineata= ANGEL. p. 66.

Fig. 37. Horizontal section of cephalic eye near the inferior margin
30/1. The dark points lie in the outermost stratum of the
cheek.
» 38. Vertical section of cephalic eye 20/1. An irregularly
hexaedral crystal below each lens. Specimen from Wisby _a_.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. V.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

Pl. VI.

=Phacops macrophthalmus= BURM. p. 65

Fig. 1. Horizontal section of the cephalic eye 30/1.
» 2. A similar one near the inferior margin, _a_ cheek 30/1.
» 3. Vertical section of a cephalic eye 30/1.
» 4. A similar, twice as much enlarged 60/1.
» 5. Also a similar, _a_ cheek 30/1.
» 6. Hypostoma 2/1. Spec. EA.
» 7. The wings of the hypostoma, seen without foreshortening
4/1. EA.
» 8. The diminutive right macula 12/1, surrounded by incoherent
terrace lines.
» 9. The left macula 12/1. Specimens from Gerolstein Eifel, Devon.

=Acaste= sp.? p. 51.

Fig. 10. The hypostoma 8/1. From Mulde, Fröjel, Gotland.

=Phaëtonides Stokesi= MURCH. p. 66.

Fig. 11. Hypostoma 4/1. Gotland, Eksta, Djupvik.

=Phillipsia= sp. p. 66.

Fig. 12. A portion of the cephalic eye 20/1.
» 13. The hypostoma 4/1.
» 14. The right macula 12/1. Specimen from Beeren Eiland, Oswalds
förberg, communicated by Hr. J. G. ANDERSSON.

=Phillipsia= sp.? p. 66.

Fig. 16. The hypostoma 3/1.
» 16. The right macula 12/1.
» 17. Horizontal section of the cephalic eye 60/1.
» 18. Vertical section of the cephalic eye, _a_ cheek 60/1.
Probably from the Carbon of Belgium.

=Proetus concinnus= DALM. p. 67.

Fig. 19. Horizontal section of the cephalic eye, a little oblique, at
right the innermost stratum with the rock 60/1.
» 20. Vertical section of the cephalic eye. The diameter of the
prisms 0,03 millim.
» 21. Hypostoma 4/1.
» 22. The right macula 12/1.
» 23. The same with a light, transparent spot 12/1. Specimen
from Djupvik, Eksta.

=Proetus conspersus= ANG. p. 68.

Fig. 24. A portion of the cephalic eye 20/1.
» 25. The hypostoma. Cast of the interior side 4/1.
» 26. Cast of the interior side of the left macula 12/1.
Fig. 24 from Öja, Gotland, the others from Kräklingbo.

=Proetus signatus= LDM. p. 68.

Fig. 27. Hypostoma 4/1.
» 28-30. Three pair of maculæ with dark ocelli in a white spot.

=Ptychopyge angustifrons= DALM. p. 69.

Fig. 31. Horizontal section of the cephalic eye 250/1.
» 32. Hypostoma, a broad variety 1/1.
» 33. Another, a narrow variety 1/1.
» 34. The left macula of the hypostoma fig. 32 4/1.
» 35. A similar of the narrow variety 4/1.
» 36. Vertical section of a macula. _a_ the macula 20/1.
Specimens from Vestanå and Kungs Norrby, Östergötland.

=Ptychopyge cincta= BRÖGGER p. 69

(according to FR. SCHMIDT. The figure 12 _a_ of BRÖGGER resembles, however, the hypostoma of Pt. angustifrons, but not the hypostomata which have been prepared from the Ptych. cincta).

Fig. 37. A hypostoma; the maculæ are directed obliquely 2/1.
» 38. The right macula of another specimen 4/1.
» 39. Vertical section of a macula 60/1. Specimens from Kungs
Norrby, Östergötland.

=Ptychopyge aciculata= ANG. p. 68.

(Compare BRÖGGER II Taf. I, fig. 13.)

Fig. 40. The hypostoma, cast of the interior surface 1/1.
» 41. Cast of the right macula 4/1. Specimen from Öland, Brunsby.

=Ptychopyge glabrata= ANG. p. 69.

Fig. 42. Original of BRÖGGER'S fig. 14. Cast of the inside of
the right macula 4/1. Ǎlleberg.

=Elyx laticeps= ANG. p. 19.

Fig. 43. The interior surface of the right fixed cheek and the
glabella. Magnified. Specimen from Andrarum. 4/1.

=Conocoryphe= p. 19.

Fig. 44. Cast of the interior surface of the left cheek. Magnified.
From Andrarum. 4/1.

=Solenopleura brachymetopa= ANGEL. p. 19.

Fig. 45. The head shield, cast of the interior surface. Magnified.
Specimen from Andrarum, Scania. 4/1.

=Apus cancriformis= SCHÄFFER p. 73.

Fig. 46. The hypostoma from the outside 8/1.
» 47. The same from the inside 8/1.
» 48. A side view of the same, 8/1. _a_ the duplicature of the
head shield, _b_ posterior integument of the hypostoma,
_c_ the ventricle, _d_ the mouth.
» 49. The right white spot at the posterior margin of the hypostoma,
seen from the outside in transmitted light 150/1.

K. Sv. Vet. Akad. Handl., Bd 34, No 8.

Lindström: Trilobites. Pl. VI.

G. Liljevall del.

Ljustryck af Chr. Westphal Stockholm.

* * * * *

Transcriber Note

Minor Typos were corrected. Inconsistencies in capitalization and abbreviations were left as printed.