Page:Ann. Lyc. Nat. Hist. 11 155-162.djvu/2

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Notes on Ceraurus pleurexanthemus.

stone that caps the upper portion of the ravine at Trenton Falls, there is a thick layer of uneven gray limestone; upon this rests a thin layer of shale and clay, varying from a quarter of an inch to an inch in thickness. This was the sea-bed where the remains of trilobites, crinoids, and other forms of animal life lay when buried by the overlying deposit, which now is a thin layer of bluish-gray limestone, one to two inches thick. Attached to the under surface of this layer, the following species of fossils have been found:

Stenopora fibrosa, S. lycoperdon, Ptilodictya acuta, P. recta, Aulopora arachnoidea, Stellipora antheloidea, Stictopora elegantula, Alecto inflata, Intricaria reticulata, three species of Heterocrinus, two of Glyptocrinus, one each of Anomalocystites and Glyptocystites, one of Stenaster, Lingula quadrata, Trematis filosa, Trematis n. s., Leptœna sericea, Strophomena alternata, Orthis testudinaria, O. lynx, Rhynchonella recurvirostra, Crania Trentonensis, Conularia Trentonensis, Asaphus gigas, Calymene senaria, Ceraurus pleurexanthemus, Acidaspis Trentonensis, Acidaspis n. s., Proetus parviusculus, Phacops callicephalus, Dalmanites.

These fossils are generally found in groups of associated species, but often commingled, so that trilobites, crinoids, cystids, brachiopods, and bryozoans occur on the same slab of stone. The prevailing and characteristic fossil is Ceraurus pleurexanthemus. Individuals from three-sixteenths of an inch to two inches in length, are scattered over the surface, often to the exclusion of all other fossils. In a space thirty by forty feet, 326 entire specimens were seen. Of this number, and of many seen before, a record was kept; eight lay with the dorsal surface up; the remainder were on their backs, attached by the ventral surface of the dorsal shell to the under side of the layer. The view that this was the natural position of the trilobite is sustained by the following considerations:—

1. Individuals of all stages of growth are preserved entire; which would not have been the case, had they been