154 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



There is an Eocene species whicli may belong to Sitiodia, but not 

 having seen specimens of it I cannot yet be sure of its affinities. This 

 is C. despecia, Desh., which is described by him as an oval shell, 

 thick and solid, becoming subtriangular with age, and the figure 

 shows that the umbones are subcentral. The hinge-plate is broad 

 and thick, and the anterior lateral is small and conical, fitting into 

 a small round depression in the right valve. If this turns out to be 

 really a Sinodia it will be the earliest species of the group. 



3. Callista, Morch, 7ion Poll. 



This genus has been accepted by most recent authors as derived 

 from Poli (1791), but is rejected by Dr. Dall on the ground that Poli 

 was not a binomial author. It is true that Poli had a double system 

 of nomenclature, i.e. one set of names for the animal and another for 

 the shell, but each was binomial. His method of nomenclature has 

 been described and discussed by me in a previous paper/ in which 

 it was shown that Poll's name Callista was applied to the animal 

 only, and that in mentioning the various species of shells he used 

 the Linnaean names, both of genus and species. Consequently I do 

 not think the name Callista can be derived from Poli. 



By some later writers, however, the name has been used in a 

 conchological sense. Thus Leach in 1852 and Morch in 1853 both 

 applied it to groups of the Linnsean genus Venus, but neither of them 

 specified a type. So far as I can ascertain, the earliest designation of 

 a type for Callista (as a genus of shells) was made by F. B. Meek ^ in 

 1876, who adopted Venus chione, Linn., and as this species formed one 

 of Morch' s Callista group the name with the type C. chione may be 

 . dated from Morch's use of it. 



Callista has an oval shell, smooth, or concentrically grooved, but in 

 the recent forms always having minute radial discontinuous ingrained 

 striae beneath the vernicose periosti^acum, especially on the posterior 

 slope. The hinge has three cardinal teeth in each valve ; in the right 

 valve the anterior and median cardinals are close together and have 

 flat opposing faces, while in the left the corresponding teeth are 

 united at the top and diverge like the sides of an obliquely written 

 A ; moreover, the median is always the thicker. In the left valve 

 there is a single prominent anterior lateral ; in the right an isolated 

 pit for its reception, bordered by a small tooth below and a smaller 

 one above. The pallial sinus is ample, horizontal, and pointed in front. 



The genus is represented in Cretaceous rocks, but none of the species 

 agree altogether with the above description, so that a distinct section 

 or subgenus will have to be created for them. 



In the Eocene series there are sj)ecies which undoubtedly belong 

 to the typical gi'oup of Callista, though some are referable to a special 

 section of it. These latter were described under the name of Callista 

 (as a section of Cytherea, Lam.) by M. Cossmann in 1886 (op. cit., 



Proc. Make. Soc, vol. viii, p. 99 (1908). 



" Cret. and Tert, Fossils of the Upper Missouri " : Rep. U.S. Geol. Surv. Terr., 

 vol. ix. 



