CAEBOXIFEKOITS SYSTEM. 271 



genera and species. But there are few if any of these forms 

 peculiar to this formation — either occurring in one or other of 

 the formations immediately above or below, or in both, as is 

 more generally the case. 



Trilobites and Ostracoids are the only remains of the 

 Aeticulata thus far known from these beds. The former are 

 represented by the single genus PMllipsia, and the latter by 

 Beyricliia and Cy there. 



Yeetebrates are only known by the remains of fishes. 

 These comprise the teeth and dorsal spines of Selachians, or 

 sharks, and Ganoids. The latter consist of detached scales 

 belonging to the genus Palceoniscus, or to a closely allied 

 genus, and rarely fragments of the internal skeleton. Among 

 the sharks the following genera have been recognized: Clado- 

 dus, Orodus, Relodus, Petrodus, Deltodus. With our present 

 imperfect knowledge of the distribution of the fishes in the 

 coal-measure series, it would be difficult to say what forms are 

 peculiar to any one of its members. On the other hand, it is 

 well known that there are forms which range from the Lower 

 measures up into the upper beds of the Upper formation. 

 The genus Petrodus, for example, is represented apparently 

 by the same species in all three members of this group ; and 

 the Peripristis semicircular is is known from the Lower and 

 Upper formations, though it has not been detected in the 

 Middle formation. Of the Ganoids, the genus Palceoniscus is 

 represented by the same or very closely related species in 

 each of the formations. 



These remains are of rare occurrence in this formation, and 

 do not seem to mark any particular horizon. However, in 

 most of the carbonaceous shales Petrodus occurs, and almost 

 invariably associated with it is a peculiar dorsal-like spine, 

 which we are inclined to regard as having belonged to the 

 fishes, with whose dental remains they are so generally found. 

 The species are usually about an inch in length, regularly 

 tapering from the base, which is about one-fifth of an inch in 

 width, to the apex, with fluted surfaces, the ridges of which 

 terminate on the posterior margin in delicate spiny processes. 



