86 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



CYCLAS, Bruguiere. 



Testa ovato-globosa, sequivalvis, subaequilateralis, tenuis, epidermide oli- 

 vacea induta ; dentibus cardinalibus minimis in utraque valva duo- 

 bus, uno bifido ; lateralibus in valva altera binis, subelongatis, in 

 altera quatuor, duobus perexiguis. Ligamentum externum. Im- 

 pressio muscularis pallii sinu nullo. 



The genus Cyclas was instituted by Bruguiere for the reception of a 

 small group of freshwater Tropiopoda, distributed by the early natural- 

 ists in the genera Tellina, Chama, &c. The Cyclades, however, then in- 

 cluded the Cyrena, but Lamarck saw the propriety of separating them ; 

 and although De Blainville now holds to the original arrangement of 

 Bruguiere, he at one time adopted the divisions of Lamarck as sub- 

 genera, under the titles of Corneo- Cyclas and Cyano-Cyclas. 



The shell of Cyclas may be described as being ovately-globose, equi- 

 valve, and nearly equilateral ; it is moreover thin, and covered with an 

 olivaceous epidermis. There are two cardinal teeth in each valve, one of 

 which is bifid ; with two lateral teeth in one valve, and four in the other, 

 two of which are very small. The ligament is external, and there is no 

 sinus in the muscular impression of the mantle. 



Some of the Cyclades are furnished with an elongated siphon, and 

 have been separated by Pfeiffer under the generic title of Pisidium. 



Example. 



PI. LXII. Fig. 1 to 3. 



Cyclas rivicola, Lamarck, Anini. sans vert., new edit., vol. vi. p. 267. 



Turton, Manual of British Shells, pi. 1. f. 1. 

 Tellina cornea, Dillwyn. 

 Cyclas cornea, Draparnaud. 



