DIODONTA. 283 



entire ; the hinge-margin moderately wide ; and the palleal sinus 

 very large. The teeth, which are obscure and very small, are 

 stated by Montagu, who had probably the opportunity of exa- 

 mining them in a more perfect state than we had, to consist of 

 two approximate sub-bifid ones, interlocking with the solitary 

 one of the opposite valve. The hinge-margin of the right valve 

 is a little channelled in front, where it becomes slightly broader ; 

 there are no lateral teeth. The length of a large single valve 

 was eight-ninths of an inch, and its breadth all but half an inch ; 

 these are, however, beyond the average dimensions. Introduced 

 by Montagu, as dredged h>j LasJcey in the Frith of Forth : accord- 

 ing to the late Mr. Humphreys, it is an inhabitant of the shores of 

 the West Indies. 



DIODONTA, Deshayes. 



Shell subequivalve, inequilateral, both valves convex. 

 Surface more or less striated or ridged transversely, not 

 invested with an epidermis. Hinge composed of two pri- 

 mary teeth in one valve and one bifid tooth in the other ; 

 no lateral teeth. Ligament external. Muscular and pal- 

 leal impressions strongly marked. The former oblong and 

 nearly equal ; the latter with a very deep and wide sinus. 



Animal ovate ; mantle open, its margins fimbriated ; si- 

 phons separated to their bases, unequal, their orifices 

 fimbriated. Foot linguiform. Branchia) equal. Labial 

 tentacles triangular, rather large. 



The animal of B. fragllis has been examined by Philippi 

 and by Deshayes, both of whom have given figures of it. 

 The mantle is freely open in front, and, like the rest of the 

 body, of a yellowish white colour. Its margin is fim- 

 briated. The siphons are separated throughout their length 

 and very unequal, the branchial one much the longest, and 

 having a corrugated brownish base. The branchial orifice 



