198 MOLLUSCA FROM THE CRAG. 



three principal teeth in the left valve, with one distant, angular, and somewhat pro- 

 minent lateral tooth in each ; fulcrum for the ligament long and prominent, and in the 

 old specimens of this species may be seen the oblong cavity before the ligament, while 

 in the younger shell the fulcrum extends nearly to the umbo. The surface is covered 

 with rather prominent and distinct lines of growth, and there are fine, radiating, and 

 punctated striae, but they are not always visible even with a lens, and were probably, 

 like those upon Isocardia cor, most distinct in the epidermis. It is very variable in 

 its outward form, some specimens being transverse and very inequilateral, while others 

 are very orbicular, with a height rather exceeding the length ; outwardly it so much 

 resembles Isocardia cor as evidently to denote a close relationship. 



CIRCE.* Schum., 1817. 



VENUS (sp.) Mont. 

 CYPRINA (sp.) Turt. 

 CYTHEKEA (sp.) Phil. 



Gen. Char. Shell equivalve, subtrigonal, orbicular or ovate, thick, strong, and 

 closed ; umbones sharp, not much elevated ; lunule distinct, margins smooth. Hinge 

 with three primary teeth in each valve, and one anterior lateral tooth. Ligament 

 external ; palleal impression with a very small sinus. 



Animal suborbicular, with its mantle open, and the margins denticulated ; two 

 short siphonal tubes, with fringed orifices ; foot large and tongue-shaped. 



The authors of the ' History of British Mollusca' have adopted Schumacher's genus 

 for the living homologue of our Crag species, although with some doubt respecting the 

 propriety of the reference. The main features of distinction are the short or sessile 

 appearance of the syphons, which correspond in that respect with those of Cyprina and 

 Astarte, and they have placed it between those genera ; the dentition of our only 

 species resembles that of Cytherea, and if it does not strictly belong to the latter genus, 

 deserves a situation but little removed from it. 



1. CIRCE MINIMA. Montague. Tab. XIX, fig. 2 a d. 



VENUS MINIMA. Mont. Test. Brit, p. 121, t. iii, fig. 3, 1803. 



TRIANGULARIS. Mont. Test. Brit., p. 577, t. xvii, fig. 3, 1803. 



CYBILLI. Scacchi. 1832, fide Philippi. 



PUMILA? Desk. 2d edit. Lam., torn, vi, p. 370, 1835, No. 86. 

 CYPBINA MINIMA. Turt. Brit. Biv., p. 137, 1822. 



Thorpe. Brit. Mar. Conch., p. 82, fig. 102, 1844. 



* Etym., proper name. 



