158 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



as well as in the larger size of some of the species, to Batissa, but differs. in 

 having shorter, and smooth, instead of striated, lateral teeth. From Velorita, 

 it will be distinguished by the more gibbous, obliquely-cordate form of the 

 latter, which also differs in having its lateral teeth finely striated, and the 

 anterior one more nearly approaching the front cardinal tooth. All three of 

 the cardinal teeth in Velorita are also directed more obliquely backward than 

 we usually see in Cyrena. 



Several species having the form and external characters of Cyrena have 

 been described as such from Jurassic and older rocks ; but as their hinges 

 and internal characters are unknown, it remains to be determined whether 

 they belong to this genus or not. Many species have also been described 

 under the name Cyrena, from the Wealden of England and Germany ; but 

 at the time these species were published, by Sowerby, Roemer, Dunker, and 

 others, little or no attention was generally paid to the distinctions between 

 Cyrena and Corbicula ; all such shells being then (especially to palaeontol- 

 ogists) known under the name Cyrena. So far as can be determined from 

 the published figures of these shells that show the hinge, they seem all to 

 have the elongated, linear, lateral teeth of Corbicula, and consequently 

 cannot, with possibly a few exceptions, belong to the genus Cyrena as now 

 understood by conchologists. It is also worthy of note that these Wealden 

 species are generally figured, and indeed have been described by Dunker, as 

 if they have but two instead of three cardinal teeth to each valve. If there 

 is no mistake about this, they could not be properly included in Cor- 

 bicula, or any of the other described genera of this family ; but would 

 constitute a distinct and unnamed genus. As has been remarked by Sto- 

 liczka, however, it is possible that they may have the third anterior cardinal 

 tooth of the right valve, and posterior of the left (which in Corbicula, are 

 often very small), present in a rudimentary condition, and thus not differ 

 generically from that group ; or possibly, in some cases, they may fall into 

 the genus Velorita, with which some of them nearly agree in form. 



The genus Cyrena was undoubtedly represented during the Tertiary 

 epoch, and probably attains its greatest development at the present time. The 

 species are found in the brackish waters of tropical regions, in Central and 

 South America, Australia, India, Africa, China, Islands of the Pacific, &c. 

 Both of the sections here recognized were represented during the deposition 

 of the Tertiary rocks, and still. exist. 



