250 TWENTIETH REPORT ON THE STATE CABINET. 



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 diiferences 

 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 thickness 

 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 

 little extension of the characters of Productus or of Strophalosia might 

 include them ; but while Strophalosia and Aulosteges remain distinguished 

 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 Productidae. 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 having narrow hinge- 

 areas and small teeth and sockets, with reniform vascular impressions of 

 the form of those illustrated, the name Productella. 

 Subgenus Productella (n. s.g.). — Shells having the general form of 



Productus^ but uniformly with a narrow area on each valve, a foramen or 



callosity on the ventral area,! small teeth, and more or less distinct 



teeth-sockets. 



The reniform vascular impression, rising from between the anterior and 

 posterior occlusor muscular impressions, curves gently outwards, and, fol- 

 lowing a curvature somewhat parallel with the margin of the shell to below 

 the middle of its length, is abruptly recurved, and the extremity, turned a 

 little backwards, terminates about half way between the margin and the 

 anterior extremity of the mesial septum. 



The cardinal process, seen from the inner side, is bilobed, and from the 

 exterior side each of these divisions is usually bilobed. 



These shells differ from Strophalosia in the extremely narrow linear 

 cardinal area, greater extension of the hinge-line, more extreme arcuation 

 or ventricosity of the ventral valve in many or most of the species, and 

 especially in the direction and termination of the reniform vascular 

 impressions, which resemble those of Aulosteges and of some species of 

 Productus. It differs from Productus in the constant presence of an area, 

 hinge-teeth and sockets. 



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

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

 absence of teeth and teeth-sockets are the distinguishing features. 



I In the specimens affording the most satisfactory evidence of area, etc., the foramen 

 is open ; but in some specimens the impression only remains, and there is the appearance 

 of a narrow indentation below the beak, so that it is impossible to determine whether the 

 foramen has been open, or closed by a deltidium. 



