ural position of the living animal, which must, therefore, have been with the back downward.
The Asaphus is more frequently broken; but the finest and most perfectly preserved specimens, with but few exceptions, are found on their backs.
That portion of the fourth conclusion in reference to trilobites living gregariously in vast numbers, is true of Ceraurus pleurexanthemus, Asaphus gigas, and A. megistos, as found in the stratum mentioned.
Note. To October 16th, 1875, 1160 specimens of Ceraurus pleurexanthemus have been noted on the under surface of the thin layer ("Ceraurus layer"). Of these 1110 lay on their backs; while but fifty presented the dorsal surface up. Forty-five of these fifty were very small, the remaining five of medium size.
XVIII.—Description of the Interior Surface of the Dorsal Shell of Ceraurus pleurexanthemus, Green.
ВY C. D. WALCOTT.
Read June 7, 1875.
This interesting species, which has been referred to in the preceding pages, has already been described by earlier writers, as regards the general features of its structure and the outer surface of its shell. In this article, therefore, I shall omit all detailed reference to any of these points, and confine the description, as closely as may be, to the inner, or ventral, surface of the dorsal shell. This description is de-
- Green, Monograph of Trilobites, 1832, page 84. fig. X. Hall, Palæontology N. Y., vol. I, page 242.