153 
PALAEONTOLOGY OF NEW-YORK. 
and in that view Productus 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 Productus 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 Productidse, 
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 Strophalosia and Auloste- 
ges from Productus 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 
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 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 sufficient to. indicate a distinct group; and a 
* The disposition of the reniform impressions is not essentially different ftom that of some species of 
Psoductus ; and the presence of an area and covered foramen, and the absence of teeth and teeth-sockets 
are the distinguishing features. 
