472 ungulinidjE. 



Tn Scacchia the foot is compressed and tongue-shaped, 

 instead of being vermiform and tubular, as in the preceding- 

 genus. The species at present described are from the Me- 

 diterranean. 



Species of Scacchia. 

 elliptica, Scacchi. ovata, Phil. 



Genus MYSIA, Leach. 



Mantle-margins nearly plain ; pedal and anal apertures 

 wide apart. 



Shell equivalve, regular, more or less sub-orbicular, ine- 

 quilateral, nearly smooth, closed. Hinge composed of two 

 cardinal teeth (the posterior of the right, and anterior of 

 the left valve bifid) in each valve ; lateral teeth none ; 

 ligament double, rather long, sub -marginal. Muscular 

 impressions roundish, nearly equal ; pallial line without a 

 sinus. 



Syn. Diplodonta, Bronn. Sphserella, Conrad. Mit- 

 trea, Gray. 



Ex. M. Brasiliensis, Philippi, pi. 114, fig. 6. Shell, 

 M. rotundata, Montagu, fig. 6, a, G, b. 



In this genus the labial palps are fully developed, large, 

 and membranous; the gills are composed of two lamina? 

 on each side ; the siphonal orifice is single; and the foot is 

 thickened at the end and perforated, and when not pro- 

 truded from the shell is folded up between the gills. The 

 foot of M. lupina is said to be compressed and linguiform, 

 on which account Dr. Gray has limited this genus to that 

 species, giving to the other species the name of Mittrea. 



