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.[36]