TRANSACTIONS OF WAGNER 

 1444 



^ TERTIARY FAUNA OF FLORIDA 



very faintly striated, but are never sharply crenate, as in Corhicula. The liga- 

 ment is external and short, usually rather prominent ; the opposing surfaces 

 of the conspicuous nymphs are sometimes granular or rough. The inner 

 margins of the valves, sometimes feebly striated, are never crenate or fluted. 

 The exterior is smooth or concentrically sculptured, the sculpture being usually 

 of small, sometimes sharp-edged, waves. These are sometimes confined to 

 the anterior half of the disk. There is a conspicuous brown or olivaceous 

 periostracum, and the interior of the valves is porcellanous, white, or suffused 

 with violet, salmon color, or pale orange. The lunule is sometimes indicated 

 b}' an impressed space more polished than the rest of the surface, but rarely if 

 ever circumscribed by an incised line ; there is no defined escutcheon and rarely 

 any distinct lunule. 



Cyrena is said to occur as early as the Jurassic. The earliest typical forms 

 I have seen are from the Cretaceous, and not lower than the middle of that 

 epoch, but I have not been able to examine many fossil pretertiary species. 

 All the earlier types appear to have a visible pallial sinus. I suspect Corbicula 

 to be the older type of the two, as it is by far the most prolific both in types 

 and species. 



The group may be divided as follows : 



Genus Cyrena Lamarck. 



Subgenus Cyrena s. s. (see above for characters). 



Section Cyrena s. s. Type C. bengalensis Lamarck. 

 Pallial line entire or only a little truncate below the posterior adductor 

 scar ; shell rounded-trigonal, plump, usually large. Oriental seas at the 

 mouths of rivers, sometimes in permanently fresh water. 



Section Geloina Gray. Type C. coaxans Gmelin (= C. seylanica La- 

 marck) . 

 Pallial line with a minute nearly obsolete sinus close to the adductor scar. 

 Warmer regions of the Old World. 



Section P olymesoda Rafinesque. Type C. caroliniana Bosc (= C. caro- 

 linensis Lamarck) . 

 Pallial line with a deep, narrow sinus, otherwise like Cyrena s. s. In streams 

 of America in the warmer regions. 



Subgenus Pseudocyrena Bourguignat. Type C. ciibensis Prime (-{- C. mari- 

 tima Orbigny, not C. B. Adams). 

 Shell small, thin, ovate or rostrate, inequilateral, the pallial sinus very 



