CONTRIBUTIONS TO PALAEONTOLOGY. 287 



period has acquired for it the importance and distinction which it must 

 retain ; and any subdivisions proposed must have reference to the char- 

 acters of this genus as established. 



On a critical examination of the genera of the Family Productidse, we 

 shall find that Chonetes is distinguished by well-marked and important 

 characters, having an area on each valve and a row of spines upon the 

 margin of the area of the ventral valve, which is one of the distinguishing 

 external features, though it has been ascertained that spines sometimes 

 occur on the body of the shell. 



The separation of the genera or subgenera Strophalosia and Aulos- 

 TEGES from Produotus is based upon less important differences. The 

 former has an area on each valve, a foramen in the ventral valve covered 

 by a deltidium, with teeth and sockets, while the reniform vascular 

 imprints have a different direction and termination. The latter has a 

 wide area on the ventral valve, a foramen closed by a pseudo-deltidium, 

 without teeth or teeth-sockets, and with the reniform vascular impressions 

 extended far towards the anterior margin of the valve, and sometimes 

 abruptly recurved.* The typical species of Aulosteges is very similar 

 to Strophalosia in external form. Both are from the Permian system, 

 and the differences between them consist in the presence or absence of 

 teeth and sockets, and the different direction of the reniform impressions. 



While the typical and fully developed forms of Strophalosia in the 

 Permian system have a large area on the ventral valve, with a narrow 

 area on dorsal valve ; all the New York species have a very narrow area 

 on each valve (often so narrow as to be no greater than the thickness 

 of the shell), with the other general differences pointed out. These 

 features alone might not be sufficient to indicate a distinct group ; and a 

 little extension of the characters of Productus or of Strophalosia might 

 include them ; but while Strophalosia and Aulosteges remain distin- 

 guished by such small differences, these forms, also, I conceive, should 

 be separated from the former, both on account of the external differences 

 and from the reniform vascular impression, which has been regarded as 

 an important feature in the Productidse. These differences I consider as 

 scarcely less important than those upon which the other subgenera have 

 been separated ; and from their external form and internal structure so 

 nearly resembling Productus, I would propose for the Devonian species 



* The disposition of the reniform impressions is not essentially different from that of some 

 species of Productus; and the presence of an area and covered foramen, and the absence of teeth 

 and teeth-socliets are the distinguishing features. 



