156 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 de- 

 posit, 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 : 



Stenoptora fibrosa, 8. lycoperdon, Ptilodictya acuta, P. 

 recta, Aulopora arachnoidea, iStellipora antheloidea, >Slictop)ora 

 elegantula, Alecto injiata, Intricaria reticulata, three species 

 of Heterocrinus, two of Glyptocrinus, one each of Anomalo- 

 cystites and Glyptocystites, one of Stenaster, Lingula quad- 

 rata, Trematis filosa, Trematis n. s., Leptaina sericea, Stro- 

 phomena alternata, Orthis testudinaria, O. lynx, Phynchonella 

 recur virostra, Crania Trentonensis, Conidaria Trentonensis, 

 Asaj)hus gigas, Calymene senaria, Ceraurus pleurexanthemus, 

 Acidaspis Trentonensis, Acidaspis n. s., Proetus parvius- 

 culus, Phacops callicephalus, Dahnanites. 



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 en- 

 tire ; which would not have been the case, had they been 



