Part 10

The abdomen and post abdomen are not distinct. The abdomen exclusive of the cauda is three inches and a half long; it exhibits fourteen costæ varying indiscriminately from one-fifth to one-fourth of an inch in breadth, except the three inferior ones, which are rather broader; they occupy the whole abdomen without membranous interspaces, and are separated by a black sulcus, not always well defined, and sometimes a line in diameter. Each costa is canaliculated from the upper and under angle to the tip.

The middle lobe is separated from the lateral by a shallow, rude sulcus, which however, does not always destroy the continuity of the costæ, as they cross it; this lobe is slightly convex, one inch and a half broad at the top, and so continues to the sixth costa, after which it gradually contracts, until at the bottom it is one-fifth of an inch broad, subsiding insensibly into a flat membrane-like surface; its longitudinal sulci pass one inch farther downwards, and expanding a little, unite with the costæ on each side the posterior edge of the space included by them, being dentated.

The lateral lobes are quite flat, one inch and a half broad anteriorly, and, by gradual prolongation, become at the fourth costa one inch and four-fifths in breadth; this dimension is maintained to the ninth articulation, when it slowly decreases to one inch at the bottom; the recurvature of the costæ is gentle in the upper eight, but then decreases rapidly. Their extremities, advancing two-fifths and four-fifths of an inch into the embedding rock, are falcate with their raised black edges, and clearly marked points.

This trilobite was found by Lieut. Bolton, at Lockport, in the state of New York, in the black, shaly, horizontal limestone forming the lower part of the ravine by which the Erie canal ascends the parallel ridge of Lake Ontario. Dr. Bigsby remarks on this locality, "I am not prepared to assign to this limestone its exact place in the series of geological formations; it is above the saliferous sandstone, and therefore more recent than the rocks best known as abounding in trilobites." We have, therefore, in this instance, another fact, which demonstrates that blind trilobites are not confined to a geological period more remote than That which has produced the animals with oculiferous tubercles.

Genus Ogygia. Brongniart.

In the vast quarries of slate at Angers, in France, there is frequently noticed two very remarkable organic remains, which have for a long time excited the attention of naturalists. To receive these curious relics, Professor Brongniart established the genus, Ogygia, which he thus characterizes. Body much depressed--elongated into an ellipse, terminated in points--nearly equal at its extremities, and not capable of contracting itself into a spherical form. The buckler is bordered by a slight longitudinal furrow, rising from its anterior extremity, and its posterior angles terminating in elongated points. The abdomen has eight articulations, and its longitudinal lobes are not very prominent. The eyes are neither prominent nor reticulated and there are no other protuberances on the buckler.

In Professor Brongniart's original work on the Trilobites, he has described the two specimens from Angers, under the specific names of Guettardi, and Desmarestii, in compliment to M. M. Guettard and Desmarest; and in De La Beche's Manual of Geology, we are informed that he has since identified two other species; one of which is supposed to be found in North America; this he calls Ogygia Sillimani; the banks of the Mohawk River, near Schenectady, is the locality from which it is said to have been derived.

As we have not been able to find any detailed account of this species, we have admitted both it and the genus to which it is said to belong into our Monograph exclusively on the high authority of Professor Brongniart, as quoted in the Manual of Geology. We are not ignorant of the species of Trilobites found near Schenectady, and if permitted to offer a suggestion on this subject, it would be, that the description of the American ogygia, was made out by its distinguished author from the fragment of an Isotelus. The Isotelus is not uncommon in that vicinity, and one of its extremities might, even by a very close observer, be mistaken for that of an Ogygia--especially by those who are not perfectly familiar with the Isotelus.

Genus Isotelus. Dekay.

This fine genus of trilobites was established November, 1824, by my friend James E. Dekay, M. D. It embraces a considerable number of species so analogous to each other, that except in a very few cases, it is exceedingly difficult to point out their distinctive characters. Some of the species of Isotelus, appear to have reached a greater size than any other trilobite. In the cabinet of P. A. Browne, Esq., there is the fragment of one, which must have been fourteen or fifteen inches long.

The Isotelus is found in several parts of North America, but most abundantly in the black transition limestone, in the northern section of the country. The richest locality, not only of this genus, but also of the Calymene and the Asaph, is Trenton Falls, on West Canada Creek, about 13 miles to the north of Utica, in the state of New York. The following extracts from the notes of Professor Renwick, which accompany Dr. Dekay's account of the Isotelus, will give some idea of this vast depository of the medals of ancient zoology. West Canada Creek, is one of the principal branches of the Mohawk River. At Trenton Falls it has worn itself a passage through the rock for the distance of nearly two miles, forming a series of water falls; and has thus laid open to view the strata to the depth of probably 300 feet. The layers of the rock thus disclosed are nearly horizontal, and of various thicknesses: they are composed of limestone, with the exception of numerous thin veins of argillaceous matter. The higher strata are composed of carbonate of lime nearly pure, of a light grey colour and crystalline structure. At greater depths it is more compact and darker in colour, and finally it appears quite black and highly fœtid.

Animal remains are contained in every part of the rock; besides several genera of trilobites, we have several species of orthocera. Encrinites and Fungites--Nautili--Terebratulæ and Producti, are quite common. The favosites here are sometimes six inches in diameter, and in such numerous columns, as to have induced the late worthy proprietor of this interesting spot, Mr. J. Sherman, to consider them as analogous in structure to the basaltic columns of Staffa and the Giant's Causeway; he therefore maintains the extravagant theory that these columns are nothing more than gigantic favosites. We visited this famous locality of trilobites not long since, and were almost as much delighted with the sublimity and grandeur of the cataract, and the picturesque and romantic character of the glen, as with the reliques of olden times, which are scattered here in such profusion.

The genus Isotelus, derived from Ἱσος, equal, and τελος, extremity, is thus characterized by Dr. Dekay.

Body oval often contracted, not unfrequently extended.

Head or buckler large and rounded, equalling the tail in size, but with two oculiform tubercles.

Abdomen with eight articulations.

Frontal process beneath, with two semilunar terminations.

Post abdomen or tail, broad, expanded with indistinct divisions, as large as the buckler.

Longitudinal lobes very distinct.

Other distinguishing marks by which this genus may be known, have been given in our introduction.

Isotelus Gigas. Dekay. Casts Nos. 21 and 22.

Head representing a spherical triangle, surface punctate, convex, descending from between the eyes to the anterior border, which has a narrow raised rim; posterior extremity concave and corresponding to the articulation of the abdomen. Eyes elevated, prominent, sub-pedunculated; cornea oblong, lunated, highly polished; abdomen with eight distinct articulations, the middle lobe double the size of the lateral one: these latter are continuous with the middle lobe, have a deep furrow impressed on their upper surfaces, which becomes gradually effaced towards their narrow free extremities. These lateral lobes are rounded at their extremities, and flattened in such a manner as to allow each lobe to slide easily under the lobe immediately preceding. Tail subtriangular, convex, equalling the head in size, with the posterior termination rounded. On the centre of its surface, when accidentally decorticated, a slight elevation may be traced, if the specimen be held in a certain light, which appears to be a continuation of the middle lobe; this extends to within a short distance of the posterior angle of the tail, when it is either entirely effaced or terminates in an abrupt truncation. Another elevation runs parallel to and at a short distance from the edge of the tail. These elevations are connected by obscure parallel lines, imitating the spaces between the lateral lobes. When the tail is fractured on the borders, a semilunar depression is visible, exhibiting concentric striæ. The whole Surface of the animal has a jet black polish. Length from 6 to 12 inches.

The original of our cast is in the cabinet of J. P. Wetherill, and was found near Cincinnati, Ohio. It is of a yellowish colour, and occurs in argillaceous slate. Specimens are common in most cabinets of American fossils. The Lyceum in New York, possesses a fragment of an individual of this species, which must have been at least 17 inches long. Our cast, No. 22, is from the gigantic tail in the cabinet of P. A. Brown, Esq. Mr. Stokes describes the I. gigas as a new species under the name of Asaphus Platycephalus, in Geolog. Trans. vol. i. N. Series. His specimen was found in the limestone of. St. Joseph's, Canada.

Isotelus Planus. Dekay. Cast No. 23.

Head more rounded than the preceding, and less elevated. Tail flat, rounded. Total length two inches and one-tenth. Breadth one inch and one-tenth. Length of the head, six-tenths--of the abdomen, eight-tenths, and of the tail seven-tenths.

Dr. Dekay is of opinion that this species may possibly prove to be the young of the preceding. The relative proportions of its buckler and tail vary considerably from those of the I. gigas; and the depth of the lateral lobes, which exceeds three-tenths of an inch, would almost of itself determine it to be a new species. The original, from which our cast was taken, is in the cabinet of J. P. Wetherill. It was found near Newport, Kentucky, and occurs in argillaceous slate. The fossil is of a dirty yellow colour.

Isotelus Cyclops. Green. Cast No. 24. Fig. 7.

Clypeo antice attenuato, plano; oculis rotundis, proximis; cauda ovata, acuminata.