478 leptonidjE. 



central excision in one valve ; a primary apical tooth in 

 front of a sub-triangular excision of the hinge-margin, and 

 flanked on each side by a sub-lateral lamina in the other. 

 Pallial impression simple. 



Ex. L. squamosum, Montagu, pi. 115, fig. 1. Shell, 

 L. squamosum, fig. 1, a, 1, b. 



In Lepton the mantle is much produced beyond the 

 margin of the shell, and is furnished with slender, tentacu- 

 lar filaments. The foot is folded as in Area, but when 

 the animal is in motion it forms an expanded disk. 



Species of Lepton. 



anomalum, Desh. nitidurn, Turt. 



Clarkise, Clark. squamosum, Mont. 



convexum, Alder. 



Genus TELLIMYA, Brown. 



Mantle produced beyond the margin of the shell, fringed, 

 and open freely anteriorly, with only one distinct and sessile 

 siphonal orifice (anal). Foot large, broad, slightly geni- 

 culate, and furnished with a byssal groove. 



Shell oval, white ; surface of valves roughened at the 

 margins with minute points; beaks prominent, pointed. 

 Hinge composed of a cartilage-pit in each valve, and pro- 

 vided with an ossicle ; a strong, triangular, pointed cardinal 

 tooth on each side of the cartilage-pit in the right valve, and 

 two rather distant, obsolete lateral teeth in the left valve. 



Ex. T. bidentata, Montagu, pi. 115, fig. 2, 2, a. 



In this genus the hinge is furnished with an ossicle as in 

 the members of the family Anatinidce, on which account 

 Mr. W. Clark places it in that group. The peculiarities of 



