126 BuivLETlN 31 136 



the hinge from above, or laterall)^, it is seen at an angle of per- 

 haps 45 degrees. In the left valve the representative of the us- 

 uallj^ narrow little bifid tooth is two almost separate plates lean- 

 ing together above and passing well into the right valve. The 

 posterior tooth so over-locks the paddle-shaped process that it is 

 quite impossible to open the shell but slightlj;- without fracturing 

 the teeth. 



So far as we are aware this is the most vigorously developed 

 hinge dentition j^et described under the genus SphcErella. Con- 

 rad founded the genus on subvexa, a Miocene fossil from the James 

 River, Va., having only the left valve before him (Medial Tert., 

 1838, p. 18, pi. X, fig. 2). In 1871 his knowledge of the genus 

 had so increased that he described the dental conditions in the 

 right valve (Amer. Jour. Conch., vol. 6, p. 200, pi. 11, fig. 9). 

 Summing up the general characteristics he saj^s : 



Hinge of right valve with three cardinal teeth, the two anterior teeth 

 sniall, entire ; posterior tooth rather elongated, parallel with the hinge 

 margin and slightly grooved. 



The above diagnosis shows a wide difference in this genus to 

 Diplodonta or Felania. The muscular impressions are larger than 

 those of the allied genera and the posterior cicatrix is much near- 

 er the ventral margin. The posterior tooth described above may 

 be said to coalesce with the adjacent tooth at the base. The 

 left valve has one small direct cardinal topth under the apex and an 

 elongated, compressed, verj- oblique tooth, with a pit between them. All 

 the species are subsphseroidal, and constitute a group of rounded bivalves 

 with a profounder cavity than exists in any other genus. 



There is one species in the Miocene of Virginia, one in the Oligocene 

 atVicksburg, and the recent ZK««acr5(f //a, Gould, is a species of Sphcerella, 

 in which the muscular impressions are very large and unlike those in Dip- 

 lodonta. 



Conrad doubtless knew in ' 7 1 that he had been mistaken in 

 1865 (Amer, Jr. Conch., vol. i, p. 9) in referring zVz/?a/« and 

 levis to this genus ; but why he should have omitted to mention 

 his btdla, p. 138, ibidem, no one now can tell. 



Dall could not possibly have had in hand specimens from 

 which he could determine the hinge in the present species or he 

 never would have lumped it in with the Diplodont(z. (Trans. 

 Wag., Ill, p. 1179-82). 



