OF THE FAMI1.V IT N \l V 1 > 1 18. 



Cabinet of the Academy of Natural Sciences. 

 Cabinet of Mr Hyde. 

 Cabinet of Mr Phillips. 

 Cabinet of Mr Conrad. 

 Diam. •">. Length •'>. Breadth l"> inches. 



Shell rhomboidal-ovate, inequilateral, ventricose ; substance 

 of the shell thin : beaks prominent, subcarinate posteriorly, 

 eroded, undulated; ligament rather short; epidermis green- 

 ish brown, with oblique obscure rays, wrinkled; dorsal mar- 

 gin rectilinear; posterior dorsal margin nbtuselt angular: 

 posterior margin acutely angular: basal margin slightly curv- 

 ed; anterior, anterior basal and dorsal margins rounded: 

 cardinal tooth in left vahc compressed, wide, reaching be- 

 yond the cavity of the beaks, double cleft : in right valve one 

 elevated recurved tooth, which clasps the side of the opposing 

 one: lateral tooth curved, short in left valve, ami long in the 

 right, in which it is double; anterior cicatrices confluent, as 

 are also the posterior; dorsal cicatrices situated on the under 

 part of the cardinal tooth, scarcely perceptible; cavity of the 

 beaks large ; nacre white. 



'Remarks. — This remarkable species was firsl observed by 

 Mr Mason and Mr Hyde. To the kindness of the for- 

 mer 1 am indebted for the use of the line large specimen 

 figured. It i- very curious in the whole apparatus of tie 

 hinge, the teeth of which resemble in some measure the 

 Symphynota compressor herein described, from the ante- 

 rior end of the cardinal tooth to the posterior end of tin- late- 

 ral, the distance is the same in both valves, but in the left 

 valve the cardinal tooth is longest, while in the rfghl valve 

 tin- lateral tooth i> longest. The peculiar character of this 



shell is in the doubU lateral tooth being in the right valve. 

 in which it differs from all the species yel described. It most 

 resembles in general form the JHasmodonta* marginata of 



Mr Say published Ins description of the <." modonta in tin- Journal ol 



the Academy of Natural Sciences of Philadelphia, 161'!. without knowing, il is to 



