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TERTIARY FAUNA OF FLORIDA 443 



The history of the genus Cyrena is as confused as that of some of the other 

 groups of this family, and has required much study to elucidate. Species 

 were included under Cyclas Bruguiere until separated by Lamarck, who used 

 the name in 1818 for a group of bivalves which he divided into two sections, 

 o and , the former mainly equivalent to Corbicula Megerle and the latter 

 containing an Astarte and three species of Cyrena as later understood. No 

 type was mentioned. Two years later Rafinesque made the first of Lamarck's 

 Cyrense, C. caroliniana Bosc, the type of his subgenus Polymesoda, leaving 

 C. bengalensis and C. zeylanica, one of which must be taken as type. The 

 latter has been frequently . used, but it conflicts with the restricted diagnosis 

 requiring an unsinuated pallial line for the genus, and was in 1844 selected 

 by Gray for the type of his genus Geloina, which has a minute pallial sinus 

 and is confined to the Old World. In this sense Geloina was revived by Morch 

 in 1861, and therefore we must accept Cyrena bengalensis Lamarck, which is 

 figured by Delessert as having an unsinuated pallial line, as the type of the 

 genus. Many of the earlier writers used Cyrena in the sense of the prior 

 Corbicula, which, of course, cannot be accepted. The number of species which 

 belong to the restricted section without any indication of a pallial sinus appears 

 to be quite small. Most of the Old- World species have a very minute and 

 superficial impression near the adductor scar, which is due to the siphonal 

 retractors, but which in a dead or worn specimen is hardly to be traced. The 

 fossil precursors of Cyrena usually have an obvious though small sinus ; and 

 the species of the New World, except in the group of Pseudocyrena, have a 

 narrow but deep and well-marked sinus. These differences taken alone are 

 hardly generic, and intergrade pretty closely, as might be expected. 



The siphons in Cyrena are separate and rather short; the branchial siphon 

 carries some papillae around the orifice, which in the anal siphon is simple or 

 nearly so. The margins of the mantle are thin and smooth with an inner fold 

 which is parallel to the external edge and finely papillose; the margins are 

 free from below the anterior adductor to the siphonal septum. The branchiae 

 are large and well developed, united behind the foot to form an anal chamber, 

 and finely reticulate. The foot and visceral mass offer nothing peculiar, except 

 that there is no byssus or byssal groove. 



The hinge is very uniform in the group, having two short left laterals which 

 are received into sockets or between duplicate laterals in the right valve. The 

 cardinals are bifid, prominent, and strong, except the anterior right and pos- 

 terior left cardinals, which are small and entire. The laterals are usually 

 rather distant from the cardinals and may be smooth or finely granulated or 



