40 CEETACEOUS PELECYPODA 



16. Mya, Linn., 1747. Shell oblong, moderately tumid, inequivalve, surface 

 concentrically striated ; hinge in the left, or smaller, valve with a broad spoon-shaped 

 process supporting the cartilage, which lies in a corresponding pit of the other 

 valve below the beak; pallial sinus very deep; some of the species have the epider- 

 mis thickened below the beaks, representing a thin external ligament. 



In the present seas the Myidje are represented by about one hundred and thirty 

 species, of which Corhula is by far the most numerous. Prom the cainozoic rocks 

 there are about one hundred species described, in the mesozoic the number is some- 

 what smaller, though the genus Corhula always predominates, and from palaeozoic 

 beds there are hardly more than twenty species known. Thus, a gradual increase in 

 the development of the various forms is noticed from the oldest epoch up to the 

 present time. 



In the palaeozoics we only meet with Corhula, and perhaps with My a and Necera ; 

 in the mesozoics Sphenia, Corhurella, and afterwards Foromya, are added ; in the 

 cainozoic time Spheniopsis, JPleurodesma, Tugonia, and others appear, most of which 

 are still found recent, though with the exception of Corhula and some species of 

 Sphenia all belong to the series of rare shells. 



The following' species have been noticed from cretaceous deposits (see Pictet ' Pal. Suisse/ 

 4^« Ser., 3^«pt., p. 36, &c.). 



CORBULA. 



1-8. — Corhula incerta, neocomiensis^ compressa, striaUda^ elegantulay g^irgyacaj punctum, and 

 JEdwardi ; — all appear to be true Cordula. 



9. — C. Costce is probably a Forom?/a, 



10. — C. socialiSy — uncertain. 



11-13. — C. gauUina, elegans and truncata are true CorlulcB. 



14. — C. Leufroyi, Gueranger figures this species in his ^"^ Atlas Paleont. de la Sarthe/' pi. 16, 

 fig. 1), and refers it doubtfully to Sowerby^s C. elegans. The figure is not very characteristic, but 

 distinctly shows the specimen to be longer and less high than the English species described from 

 Blackdown. The former name may, therefore, be retained, until a careful comparison of both has 

 been instituted. 



15-16. — C. Goldfussiana oi Matheron is identified by D^Orbigny with his C. tnmcata (nonidem 

 Sow.). This appears rather doubtful. If one has to judge from the illustrations given of the two 

 forms, Matheron^s figure seems to represent a true Corhula, the shell being rather inequilateral 

 and globose. D^Orbigny^s figure on the contrary represents a more compressed, much less 

 inequilateral species, which is, besides, nearly equivalve and most likely belongs to Corhulomya, 

 Possibly D'Orbigny has compared the original shells and found either one or the other of the figures 

 incorrect, but had this been the case, he would most probably have recorded the fact. Should, 

 however, my suggestion regarding the generic determination of C truncata, D^Orbigny, prove 

 correct, the specific name would not require to be changed. 



Yl.—C, hifrons can hardly be identified from the short description given of it. 



18. — C. Bockschii (non Bocksii), [Crass. Bockschi olim), appears to me to be more likely a Trigo- 

 nia than a Corhula. 



19. — C. suhglohosa is rather uncertain, the shell figured by Goldfuss has not much the appear- 

 ance of a Corhula, and in fact scarcely resembles any other genus of the Mtidjs, 



