Notes on Ceraums pleurexanthemus. 155 



XVII. — Notes on Ceraums jjleurexant/iemus, Green.* 



By C. 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 observa- 

 tions 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 pleurexantJiemus. Ascqjhus and other gen- 

 era are referred to here, only as giving additional evidence 

 on the points involved. 



Ceraurus pleurexanthemus is one of the most character- 

 istic 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. S4) was founded upon specimens 

 not clearly showing all the characteristics of the genus, as subsequently known. The 

 description, however, was sufficiently accurate fur the ready identification of the 

 genus, and of the species, C. pleurexanthemus. The name should therefore stand; and 

 Cheirurus of Beyrich (1815), 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 Chei- 

 rurus 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, 1812. 



Novembkr, 1875. 12 Ann. Lyc. Nat. Hist., Vol. xi. 



