152 CEETACEOUS PELEOYPODA 



more or less coarsely striated or partially smooth, ligament thin, usually situated 

 in a deep groove, pallial sinus only indicated by the impression being truncated or 

 very slightly insinuated ; teeth strong, except the anterior and posterior cardinals 

 of the left valve, which are markedly thin. The two best known species are Cytli. 

 \Lioconclia~\ castrensis, Linn., and Cyth. \_Lioconcha\ arabica, Chem. ; they at the 

 same time represent two slightly different sections ; in the former the shell surface 

 is mostly smooth, and the posterior ridge alluded to in Dione is here more or less 

 distinctly indicated ; in the latter the surface is concentrically ribbed or coarsely 

 striated. Twenty-one species are described by Boomer. 



8^. Circe, Schuhm., 1817. Shell quadrangular or roundish, solid, compressed, 

 with flattened beaks, surface concentrically ribbed, middle cardinal teeth much 

 stronger than the others, pallial impression truncated, but not sinuated posteriorly, 

 inner edge sometimes finely crenated ; ligament situated in a deep groove. Type 

 V, scriptct, Linn. 



8^. Crista, Eomer, 1857. Shell cordate or transversally ovate, solid, more 

 or less tumid, with radiating ribs or divaricated striae ; pallial sinus very small ; 

 internal margin crenated, ligament in a groove, but distinctly visible outside. There 

 are two slightly different forms, both, no doubt, belonging to the same type. The 

 first, represented by V. pectinata, Linn., and Cyth. gihba, Lam., has strong radiating 

 ribs ; there are, however, in young stages posteriorly some obliquely directed stride 

 or ribs visible, and these indicate the divaricate striation which is so well developed 

 in the second group represented by Cyth, \_Crista] divaricata, Chem. 



The Monograph of Romer will, we may expect, contain descriptions of from 

 230-240 species, most of them no doubt excellently figured and described in detail. 

 The geographical distribution of the vjsnjerin^ is a very large one, but there are 

 some of the sub-genera which are more or less limited to certain countries, thus 

 constituting good geographical groups. The species of Venus proper generally have 

 almost the widest distribution ; they are found nearly in all seas ; those of Merce- 

 naria are to a great extent American, those of Cryptogramma either American or 

 North Australian. Chione are about equally divided between the American and 

 Indian seas. Of Cytherea proper most of the species are Indian, and the same is 

 the case with Circe, Crista, and Lioconcha ; of Caltista the largest portion of species 

 are also Indian. Tivela is divided about equally between the eastern and the Ame- 

 rican seas, some few species also occurring along the coast of Africa, &c. Dione 

 is almost exclusively an American form, while Caryatis is distributed all over the 

 world. Speaking of older fossil forms, I principally mean to refer to those occur- 

 ring in cretaceous deposits, for, as already stated, the Jurassic species areas yet very 

 little known ; they only resemble Venus-, or Cytherea-ioxm.^ in external shape 

 upon which their determination solely rests. Prom strata lower than the 

 jurassics we hardly know any species of venerin^, at least none which could 

 be with even a small amount of probability referred to the sub-family ; however, 

 the hinges of those shells are yet quite unknown. 



