GENERA PRODUCTUS AND STROPHA-LOSU. 133 



little extension of the characters of Pboductus or of Steophalosia might 

 include them; but while Strophalosia and Aulosteges remain dis- 

 tinguished by such small differences, these forms also, I conceive, 

 should be separated from the former, both on account of the ex- 

 ternal differences and from the direction of 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 impres- 

 sions 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 nar- 

 row 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 following 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 teminates about half way |)etween 

 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 direc- 

 tion 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. 



• In the specimens affording the most satisfactory evidence of area, etc.. the foramen is open; hut in 

 some specimens the impression only remains, and there is the appearance of a narrow indentation below 

 the heak, so that it ii impossible to determine whether the foramen has been open, or closed by a 

 delttdium. : '- •jo 



[ Paleontology IV.] 20 



