JUKES-BROAVNE : CRETACEOUS AND EOCENE VENERIDJE, 159 



M. Cossmann gives full descriptions both of the genus and of the 

 two species, but he does not seem to have rightly understood the com- 

 position of the hinge. He says, "the cardinal plate is thick and 

 excavated [echancree), without an anterior pit, bearing four teeth in 

 the left valve and three in the right"; he proceeds to describe the 

 peculiar arrangement of these teeth, and appears to regard all of them 

 as cardinals. In this I think he is mistaken ; it seems to me that 

 the front tooth of the left valve, which is described as being small 

 and pointed, must really be a diminutive anterior lateral, like that 

 of Cyth. Lyelli, or the recent Venus verrucosa. In these shells the 

 corresponding pit in the right valve is often so small and shallow as 

 to be nearly obsolete, and I assume this to be the case in Atopodonta. 

 ■ In the left valve what seem to be the anterior and median cardinals 

 are united at the top and are separated from the shell-margin by a 

 sulcus (as in Pitaria) ; behind these there is a simple and normal 

 posterior cardinal. In the right valve there are two teeth of a reversed 

 V -shape, one above the other, and behind these is a long narrow 

 grooved posterior tooth. This is also comparable to the arrangement 

 in Pitaria and Callocardia, but represents an earlier stage of develop- 

 ment. The lower A is the primitive element of the median tooth, the 

 upper one represents the bent lamina which would become the anterior 

 cardinal and the bridge uniting it to the posterior tooth. 



Dr. Dall appears to have perceived the true structure of the Atopo- 

 donta hinge, for in his Synopsis (op. cit., p. 353) he briefly dismisses the 

 name as a synonym of Callocardia. In such dismissal, however, I think 

 he goes too far, for though the two agree in the absence of a pallial 

 sinus, the teeth-connections are in a different stage ; moreover, in 

 Callocardia the anterior lateral is strong, elongate, and sharp, fitting 

 into a distinct pit in the right valve. The two shells are also very 

 different in shape and in thickness of substance. It will, therefore, 

 be convenient to retain Atopodonta as a distinct type, though for the 

 present it may be regarded as a subgenus of Callocardia. 



7. TiVELiNA, Cossmann. PI. VI, Fig. 6. 



This is a group of small shells separated from other species of 

 Cytherea by M. Cossmann in 1886, but very inadequately defined. 

 The following is a translation of his descriptive remarks : — " I group 

 in this section, under a new name, shells which are thin, triangular, 

 more transversely elongated than the true Tivela, more or less clearly 

 grooved (sillonnees), with a large lanceolate lunule, a short sinus, and 

 a hinge distinct from Callista. Contrary to what holds in the 

 preceding groups, the shape of Tivelina, even in one and the same 

 species, varies considerably." ^ The first species and the one which 

 he regards as the type is Cyth. tellinaria, Lam. 



From the name selected and from his reference to Tivela it 

 would seem that M. Cossmann regarded Tivelina as in some way 

 related to that genus, yet he gave no other indications of such 



^ Ann. Soc. Eoy. Malac. Belg., vol. xxi, p. 119. 



