OF THE FAMILY OF NAIADES. 429 



Cabinet of the Academy of Natural Sciences. 

 Cabinet of Mr Hyde. 

 Cabinet of Mr Phillips. 

 Cabinet of Mr Conrad. 

 Diam. -5, Length -9, Breadth 1-5 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 obtusely angular; 

 posterior margin acutely angular ; basal margin slightly curv- 

 ed; anterior, anterior basal and dorsal margins rounded; 

 cardinal tooth in left valve compressed, wdde, 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, and long in the 

 right, m ivhich 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 first observed by 

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

 mer I am indebted for the use of the fine large specimen 

 figured. It is very curious in the w^hole apparatus of the 

 hinge, the teeth of which resemble in some measure the 

 Syyyiphynota compressa. herein described. From the ante- 

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

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

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

 the lateral tooth is longest. The peculiar character of this 

 shell is in the double lateral tooth being in the right valve, 

 in w hich it differs from all the species yet described. It most 

 resembles in general form the Jllasmodonta* marginata of 



* Mr Say published his description of the genus Alasmodonta in the Journal of 

 the Academy of Natural Sciences of Philadelphia, 1818, without knowino-, it is to 



