No. 109.] 



139 



with the structure of hinge here described, it seems to me sufficient ground 

 for separation. I should not omit to mention, that in far the hirger number 

 of specimens of the S. demnsa, the centre of the area of the ventral valve 

 with the dental process is broken; and though I have a large number of 

 specimens in which the two valves are united, and also many separate valves, 

 i^ is only in one or two specimens that these dental processes of the ventral 

 valve arc perfectly preserved. This fractured condition of the area may 

 produce the impression that a foramen has once existed, from its analogy 

 with shells having that character. 



In two or three other species having entire areas, I have found internal 

 characters corresponding so nearly with those of S. demissa, that I can 

 have no doubt of their relations; and in the absence of better means of de- 

 termination, I have placed all species having the area without visible fo- 

 ramen, or deltidium, in this genus. 



In the descriptions of preceding species, it is quite possible that some 

 true Stropho?ne7ia have been included under the generic designation of 

 Strophodonta, and the language has not always been sufficiently definite; 

 the foramen being spoken of as closed when none has existed. For the 

 present the name of Strophodonta is given as a subgenus, but which I 

 regard as corresponding in value to either of the other subdivisions, Lep- 

 T^NA or Strophomena. 



Geological position and locality. This species occurs abundantly in the 

 Hamilton group of Western New- York, and at Rock Island, Illinois ; 

 New-Buffalo, Iowa, and at other localities in the same neighborhood. The 

 specimens from Iowa and Illinois are always much less convex than those 

 of New- York, many of them being depressed and nearly plano-convex in 

 form. 



Strophomena demissa. 



