67 



is so thin that the contraction of the epidermis, in 

 drying, invariably cracks it. 



I have considered this the adult of *S'. velum, per- 

 haps incorrectly, as Col. Totten, who has better op- 

 portunities of making comparisons, thinks it is dis- 

 tinct, and has described it under the name of S. bo- 

 realiSf in the American Journal of Science and Arts, 

 vol. 27, p. 366. 



IPHIGENIA. 



GENERIC CHARACTER. 



Shell equivalve, triangular, valves closed ; hinge 

 with two teeth in the right valve, united above ; ante- 

 rior one small, acute, united with the margin ; posterior 

 tooth triangular and bifid ; left valve with two teeth, 

 the anterior one subcompressed ; posterior tooth 

 linear ; muscular impressions two, distant, large ; pal- 

 leal impression, with a profound sinus, extending 

 much beyond the middle of the valve ; cartilage mar- 

 ginal ; ligament exterior. 



OBSERVATIONS. 



This genus, embracing but few recent species, has 

 been separated from the Bonax of Linnseus, to which, 



