204 CEETACEOUS PELECYPODA 



sub-genus, as stated already, for the lateral teeth of (7. solitaria are not so long and 

 more prominent than they are in typical Corbulce, to which genus Zittel suggests 

 that his species may be referable. Cy. cretacea in form more resembles Velorita 

 than a CorUcula or Cyrena ; the author mentions the occurrence of two small cardi- 

 nal teeth and on either side one elongated lateral. Besides that the same author 

 (1. cit.) quotes a Cyrena? sp, indet. from the Quader-sandstone near Sirgwitz, 

 ( ? Zirgwitz), in form resembling the former species, but possessing three cardinal 

 teeth. 



Zittel also describes from the Gosau deposits (1. cit., p. 134, &c.,) a Cyclas 

 gregaria and C. amUgua ; both are apparently species of Sphcerium, but the hinge- 

 teeth have not as yet been satisfactorily examined. 



In addition to the above species quoted from cretaceous deposits I may 

 mention Cyrena arenaria, Meek and Hayden (Meek, Check-list of cret. Am. 

 fossils, p. 13. — Cyprina id., Proc. Acad. Nat. Sc, Phil., for 1857, p. 143). 



No species of Cyrbnid^^ in fact no true brackish- or fresh-water Pelecypoda, 

 have as yet been found in the cretaceous deposits of Southern India. 



In the eocene beds we meet with true species of Cyrena^ as seen in Deshayes' 

 Description of Paris fossils and in Sowerby's Min. Conch., &c., &c. Both forms, 

 with the pallial impression distinctly insinuated and entire, occur, and besides also 

 forms which recall by their smaller thickness Batissa^ and others, possessing an 

 elongated form, like Velorita ; true Corbiculce are rare. During the miocene deposits 

 the change of the climate appears to have greatly affected the development of this 

 family. Cyrena, JSatissa, and allied forms have greatly diminished and partially 

 disappeared from the European continent, while CorUcula, Sphcerium, and Fisidium 

 increased in number, they being all fresh-water species and apparently more adapted 

 to a colder climate. In the Indian upper tertiary deposits several Cyrence occur, and 

 so also in the American. At the present time the brackish-water forms are almost 

 entirely restricted to the tropical and sub-tropical regions of the whole world, and 

 the fresh-water forms are more numerous in the former than they are in temperate 

 regions. 



The total number of species belonging to the family as at present known 

 must be nearly 300 ; of this more than one-half comes under Cyrena and allied 

 forms, which clearly shows that Dunk er's statement (Wealdenbild., p. 29,) as to 

 the number of fossil species being greater than that of the recent ones is 

 exaggerated. 



XVII. Family,— CA BDIID^, 



The animals of the typical Cardia are characterized by two separated, but very 

 short siphons with fringed orifices, and a long more or less cylindrical and generally 

 geniculate foot. The mantle is very widely open in front and below, united only 

 in one point just below the siphons. The gills are in one pair on each side, of 

 moderate size, unequal, the outer thick plume being smaller than the inner, which 



