40 PALAEONTOLOGY OF CALIFORNIA. 



example), 0.76 inch; breadth, 0.83 inch; convexity, 0.36 inch. 

 Convexity of a gibbous specimen, measuring 0.86 inch in breadth, 



0.57. 



None of the specimens of this species in the collection are in a condition to 

 show the foramen, or fine surface striaB. They give a very clear idea, however, 

 of all the other external characters, in which it appears not to vary greatly. It 

 is evidently allied, judging from Mr. Davidson's excellent figures and description, 

 to a shell from the Inferior Oolite, referred hy him with doubt to R. lacunosa, 

 Schlotheim, sp. I have not access to the figures of R. lacunosa, published by 

 Schlotheim, or Yon Bueh ; but on comparing our California species with those 

 given by Mr. Davidson, it will be found that the latter represent a shell with more 

 angular plications, and a less pointed and more incurved beak. They also repre- 

 sent the flattened space on each side the beak of the ventral valve, between the 

 umbonal angles and cardinal margin, as being wider, and the umbonal angles 

 more defined ; while the front, as seen in a side view, is much less obliquely trun- 

 cated. 



Still it is by no means improbable our shell may prove to be identical with some 

 of the forms referred to R. lacunosa; but until palaeontologists have determined 

 more clearly the limits of that species, and agree in regard to which of the several 

 types, evidently included in it by some, is to retain the name, we shall probably be 

 less liable to err, in distinguishing the form under consideration by another name, 

 than if we were to identify it with Schlotheim's species. 



It seems to be the most abundant species in the rock, and the specimens are 

 generally little distorted. 



