266 SIX NEW SPECIES 



Its nacre is uncommonly silvery. It swells considerably along 

 the posterior umbonial slope. This causes its greatest diame- 

 ter to be semidistant between the beaks and posterior margin. 



I have given a view of the right valve of this shell for the 

 purpose of exhibiting its remarkable tooth. It might at first 

 be considered as a malformation, but in the three specimens 

 which I have seen this character has been uniform. 



The calceolus approaches as nearly in its general appearance 

 to the donaciformis as to any other species. It is however a 

 thinner shell, and differs in the teeth as well as the colour of 

 the epidermis. 



2. Unio Lanceoeatus. Plate III. fig. 2. 



Test a transversirn elongatd, compressd, postice subangulatd ; valvulis 

 tenuibus ; umbonibus vix prominentibus ; dente cardinali acuto, obliquo. 



Shell transversely elongated, compressed, subangular behind ; valves 

 thin; beaks scarcely prominent; cardinal tooth sharp, oblique. 



Hab. Tar River at Tarbornugh. 



My Cahinet. 

 Professor V.nnuxem's Cabinet. 

 Cabinet of the Academy of Natural Sciences. 

 Mr Nicklin's Cabinet. 

 Peale's Museum. 

 Diam. *5, Length -7, Breadth 1-7 inches. 



Shell transversely elongated, elliptical — substance of the 

 shell rather thin — beaks scarcely elevated, decorticated — liga- 

 ment small, terminating between the beaks — dorsal margin 

 slightly curved ; posterior dorsal margin carinated ; posterior 

 margin subangular ; posterior basal and basal margins curved ; 

 anterior and anterior dorsal and basal margins rounded — epi- 

 dermis lemon-yellow and olive-yellow, with transverse lines 

 of growth, glabrous — cardinal tooth compressed, crenulated 

 and oblique — lamellar tooth straight, long and rather abrupt 

 — posterior cicatrices confluent, anterior cicatrices distinct — 



