XVII.—Notes on Ceraurus pleurexanthemus, Green.
BY С. D. WALCOTT.
Read June 7th, 1875.
The writer has had the opportunity, by his residence at the type-locality of the Trenton Limestone, to make some investigations upon the structure and habits of the trilobites of that interesting horizon. The results of these observations and studies, he hopes to present from time to time, as they shall become sufficiently definite to call for permanent record.
In the present article, it is proposed to consider certain facts of occurrence, which seem to bear upon the habits and mode of life of one of the principal species of the Trenton rocks, Ceraurus pleurexanthemus. Asaphus and other genera are referred to here, only as giving additional evidence on the points involved.
Ceraurus pleurexanthemus is one of the most characteristic trilobites of the Trenton Limestone, in numbers and distribution exceeded only by Asaphus gigas, A. megistos, and Calymene senaria. It has a wide geographical, as well as vertical, range. Entire specimens, however, are rare in most localities, the head and the hypostoma being the parts usually found. At Trenton Falls, N. Y., in the upper third of the limestone, the separated heads are found in immense numbers; in many places, the surface of the rock is nearly covered with them, while only an occasional pygidium or portion of the thorax is seen.About twenty-seven feet below the coarse crystalline lime-
- The genus Ceraurus (Green, 1832, Monograph, p. 84) was founded upon specimens not clearly showing all the characteristics of the genus, as subsequently known. The description, however, was sufficiently accurate for the ready identification of the genus, and of the species, C. pleurexanthemus. The name should therefore stand; and Cheirurus of Beyrich (1845), must be regarded as a synonym; since the objection raised to Green's figure, on the ground of its indistinctness, is not tenable. The use of Cheirurus by authors is not allowable, under the rule as to priority of date adopted by the British Association for the Advancement of Science, twelfth meeting, 1842.