not hitherto furnished a single species existing in 

 tide waters which flow into the Atlantic; and the 

 only shell of this character is the Alasmodonta mar- 

 ginata, Say. 



The shells of this genus may be divided into two 

 groups, the "winged" and "not winged," as Mr. Sow- 

 erby has proposed for the whole family of Unionid^e. 

 The former are included in the genus Symphynota, of 

 Mr. Lea, which we cannot adopt, because, as Fe- 

 russac justly remarks, "in the same family generic 

 characters should be taken from the same parts;" 

 thus the other genera are formed from the absence 

 or arrangement of the hinge teeth, whilst Symphynota, 

 without any regard to the hinge, is constructed from 

 a testaceous connexion of the valves, which Ferussac 

 observes is "almost an organic character among the 

 Acephala which have the mantle completely closed 

 near the hinge margin, at least in young shells." 



Dr. Kirtland has made some interesting remarks 

 on the sexes of this family, in which he has proved 

 them to be distinct, in the opinion of many concho- 

 logists; and the sex can be ascertained by the form 

 of the shell. Almost every species has two promi- 

 nent varieties of form, one of which is shorter and 

 more ventricose than the other, which is happily ex- 

 plained by the observations of the intelligent author 

 of the essay referred to.* 



*Vide Silliman's Journ. vol. xxvi. p. 117. 



