15^ PALAEONTOLOGY OF NEW-YORK. 



and in that view Puoductus possesses fewer characters, and in the absence 

 of hinge-area, teeth and sockets, is less complete than the forms pos- 

 sessing these appendages. Nevertheless the great development of forms 

 and number of species constituting the Genus Peoductus during the 

 Carboniferous period, has acquired for it the importance and distinction 

 which it must retain ; and any subdivisions proposed must have reference 

 to the characters of this genus as established. 



On a critical examination of the genera of the Family Productidaa, 

 we shall »find that Chonetes is distinguished by well-marked and impor- 

 tant 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 distin- 

 guishing external features, though it has been ascertained that spines 

 sometimes occur upon the body of the shell. 



The separation of the genera or subgenera StTiophalosia and Auloste- 

 GES from Pkoductos 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, with no 

 teeth and 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 

 Stkophalosia 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 Stkophalosia in the 

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

 area on the 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 thick- 

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

 tures alone might not be sufficii'nt to indicate a distinct group ; - and a 



* The disposition of the renirorm impressions is not essentially diOcrcat from that of some species of 

 Pbodoctvs; and the presence of an area and covered foramen, and the absence of teeth andtceth-sockoti 

 •re the distinguishing features. 



