272 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



There is notliiug, iu fact, about these two species of shells to mark 

 them off as more than a section of TiveJa, aud they are actually 

 Hssociated with a third species which Messrs. Cossmann and Peyrot 

 rightly regard as a typical Tivela ; this is T. triangularis (Bast.), 

 which closely resembles the recent T. dolahella. Messrs. Cossmann 

 and Peyrot have retained Grateloupia as a genus, and have placed 

 Tivela under it as a sub-genus, but this is contrary to the rule of 

 generic priority, for Link's genus was proposed in 1 807 and Desmoulins' 

 iu 1828, so that the proper arrangement is to make Grateloupia a 

 section or sub-genus of Tivela. 



Cytherioi'sis (Conrad). 



This supposed genus or sub-genus was founded on a fossil from the 

 Eocene of Alabama (United States), but it is probably only a form of 

 Grateloupia, and consequently of Tivela, for the distinction which 

 Dr. Dall makes between them is a mistake, arising apparently from 

 a miscomprehension of the dentition. He states that in Grateloupia 

 " the posterior right cardinal is fused with the nymphal rugosities ", 

 and that in Cytheriopsis it is the left posterior cardinal which is so 

 fused. As regards Grateloupia this statement is absolutely incorrect, 

 for there is a deep space in the right valve between this tooth and the 

 nymphal plate. 



With respect to Cijtheriopsis, which is the Ct/therea hydana of 

 Conrad (August, 18;33) and the Gratelupia Desmoulinsi of Lea 

 (December, 1833), the former gave no figure, but Lea gave a good 

 one ^ showing the liinge of the left valve, and this is certainly that 

 of Grateloupia, as there are three prominent cardinal teeth, and 

 several obli([ue ridges on the nymph. Whether the posterior cardinal 

 is fused with the first of these ridges, or whether there is a groove 

 between them, is of small importance, seeing that in Tivela argentina 

 it is so fused, and in T. radiata it is not. There is therefore no 

 essential difference between G. hydana and the other species of 

 Grateloupia, and consequently there is no necessity to perpetuate the 

 name of Cytheriopsis or its substitute Grateloupina (Dall). 



Conclusions. 



From the preceding notes and observations it will be seen that 

 I regard Tivela as a fairly compact genus, including the fossils which 

 have b(;en described under the names of Grateloupia and Cytheriopsis. 

 Consequently I consider that the genus ranges from the Eocene to 

 the present day. 



I have shown that the so-called accessory teeth are entirely 

 confined to the nymphal plates, that. they are merely ridges developed 

 out of the rugose sculpture of these plates, and that the hinge of 

 Grateloupia closely resembles that of some recent species of Tivela ; 

 also that Cytheriopsis does not seem to differ from Grateloupia in any 

 essential respect. 



^ " Contributions to Geology," Philadelphia, 183:5 (tract), pi. ii, fig. 33. 



