164 
this might he supposed to resemble; is said to be postice brevi, 
obliqu^ truncato/^ PI. 12. 
Por comparison I add; from the Journal of the Academy of Nat- 
ural Sciences; the following description and figure of i\T. feviS; and 
also those of a smaller and quite distinct fossil species. 
[Ntjcula l.®vis, pi. 12, right hand figure, and nuctjla concbntrica, 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. Philos. Soc. (new series) vol. 
Z,pl. 4:, fig. 4. 
Desc. Shell very oblique, oval-sub ovate, 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. 
Ohs. This species is not uncommon. The young shell which 
we have represented in the plate has a different 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- 
