CO«CHOLOGY. 61 



5. Isocardia. Three species. 



Taken from the Chama of Linnaeus on account of a pe- 

 cuharity in the shape of the cardinal teeth, and the singular 

 curvature of the umbones. 



Isocardia cor. Isocardia semisulcata. 



I. Moltkiana. 



/. cor. The heart Isocardia. PI. 12, fig. 4. 



Shell free, regular, heart-shaped, equivalve, very inequi- 

 lateral ; summits diverging, strongly recurved spirally, for- 

 ward, and outv^rard ; hinge dorsal, long, similar, formed by 

 two fiat cardinal teeth, with an elongated lateral one behind 

 the ligament, which is dorsal and exterior, diverging towards 

 the summits ; muscular impressions very distinct and rather 

 small ; slightly wrinkled longitudinally ; exterior reddish 

 chestnut colour, interior white. 



The Isocardia Moltkiana is a very rare shell, and the 

 most elegant species of this genus. 



FAMILY XL 



Arcacea. Four genera. 



1. Cucullaea. One species. 



Distinguished from the Area by the muscular impression 

 within, to one side of which is an ear-shaped testaceous ap- 

 pendage ; the shell is more trapeziform, and the hinge by 

 age becomes obsolete, giving the teeth a more horizontal 

 appearance. 



Shell equivalve, inequilateral, trapeziform, heart-shaped ; 

 beaks distant, and separated by the angular groove of the 

 ligament, which is altogether external ; hinge linear, straight, 

 with small transverse teeth, having at its extremity from 

 two to five parallel ribs ; valves marked with minute and 

 strong longitudinal striae, and sometimes one valve overlaps ; 

 margins crenulated. 



C. auriculifera. The eared Cucullaea. PI. 10, fig. 1. 

 Species navicular or obliquely heart-shaped, with decus- 



