192 BULIvKTiN 31 19a 



Lia's original description. — Shell much inflated, triangular, subbi- 

 angulate behind, nearly equilateral, very inequivalve ; right valve covered 

 with regular, large transverse ribs, which terminate at the sharp carina or 

 the umbonial slope ; left valve smooth or slightly wrinkled, with two or 

 three obscure ribs passing from the beak to the basal margin, and a linear 

 umbonial slope, posterior to which there is a deep fold ; beaks very large 

 and incurved ; tooth and pit of the left valve large ; posterior slope trun- 

 cated, furnished with a double carina on the right valve and a single car- 

 ina fold on the left. Cicatrices not distinctly impressed ; cavities of beaks 

 very deep. 



Diam. .3, Length .5, Breadth .6, of an inch. 



We prefer Idea's name for this species, for, though Conrad's 

 description was given a few months earlier, it does not differen- 

 tiate this form ^2^(^(?^<a; / in fact Conrad's figure in 1846 as cited 

 above is doubtless ^Zi^(^c'5a. As late as 1865 (Am. Jr. Conch., 

 vol. I, p. 3) Conrad fails to discriminate these two very well 

 characterized species. 



This is the most conspicuous of the Claiborne sand Cor- 

 bulas. We have seen it from no other horizon, but Dall records 

 it both from the Sabine below and the Jackson above. This au- 

 thor is correct in regarding Gregorio's C. ignota as simply the 

 smooth left valve of another species, but is evidently wrong in 

 referring it to niurchisojii. For the many figures of murchisoni, 

 (both valves are given by Gregario) show plainly he knew 

 the species well. On the other hand, Gregorio's remarks (p. 

 233) and figures show he was entirely at a loss to know what the 

 left valve oi gibbosa was like. His figures of ignota are left valves 

 of gibbosa Lea. 



This species differs from the C. wailesiaiia Har. of the Jack- 

 son Eocene by its coarser lirations, and more acute biangulation 

 posteriorly in the right valve ; more contracted posterior in left 

 valve. However, wailesiana is clearly a direct derivative of 

 murchisoni. 



It is interesting to note that while in a very young state the 

 two valves are similar in size and marking (fig. 12). Adult con- 

 ditions setting in (generally abruptly) , the left valve becomes 

 nearly smooth, shows faint radiating undulations and furrows on 

 the post-umbonal slope ; while the right valve assumes the in- 



