164 



this might be supposed to resemble, is said to be -^ postice brevi, 

 obliqu^ truncato." PI. 12. 



For comparison I add, from the Journal of the Academy of Nat- 

 ural Sciences, the following description and figure of W. Isevis, and 

 also those of a smaller and quite distinct fossil species. 



[NucuLA L^vis, pi. 12, right hand figure, and npcula concentrica, pi. 

 12, left hand figure, being fossil species, I have not given the descriptions, 

 J. A., iv. p. 141.— Ed.] 



Unio ellipsis. Lea. — Specific character. Shell very oblique, 

 suboval ; summit prominent, as long as the posterior side. 



Unio ellipsis, Lea. Trans. Amer. Phtlos. Soc. (new series') vol. 

 B, pi 4, fig. A. 



Desc. Shell veiy oblique, oval-subovate, ventricose, ponderous : 

 disk convex, lines of growth deeply impressed, somewhat undulating 

 the surface ; yellowish-olivaceous, more or less radiate with green, 

 particularly before ; never spotted ; the old shell uniform brown or 

 fuscous : margin regularly arquated, even at the anterior tip : sum- 

 mit prominent, as long as the posterior side : posterior side re- 

 markably short : cavity of the hinge membranes profound, much 

 dilated between the summits and not extending posteriorly beyond 

 the line of the summits ; visible portion triangular, its posterior 

 line transverse, slightly arquated, widest : within white, perlaceous, 

 iridescent before : teeth large, prominent, the cardinal anterior of 

 the right valve and both of the left valve parallel to the lateral 

 teeth, which are a little arquated, those of the right valve united 

 at tip and even after the union a little more elevated than the 

 surrounding surface : sinus of the cartilage, regular, definite : 

 cavity of the umbo not deep, its muscular impressions very distinct : 

 posterior muscular impression remarkably deep, subobconic : palleal 

 impression anteriorly arquated, hardly extending beyond the line 

 of the muscular impression. 



Obs. This species is not uncommon. The young shell which 

 we have represented in the plate has a difibrent coloring from the 

 old specimen, which is of a uniform dark, or even blackish-brown, 

 the radii being entirely invisible. It varies in its radii, some being 

 radiated over the whole surface, others having a capillary radii 

 only on the anterior side and others being entirely destitute of 

 radii at all ages. Another variety is tinged with rosaceous, especi- 



