244 PALEONTOLOGY OF NEW JERSEY. 



observable even in their very imperfect condition, when compared with 

 their living representatives, which are certainly as great as differences be- 

 tween many of the western Cretaceous Unionida3 and the living represent- 

 atives of the same species ; and I am not at all prepared to say that any 

 one of the tvv^elve species named, if it could be obtained in condition perfect 

 enough for complete comparison, would not prove quite distinct from the one 

 whose name it bears. I have made comparisons with these living forms, 

 some of Avhich are noted under the descriptions of the species. 



The condition of preservation of these shells is a gi'eat drawback to 

 their satisfactory study. Their remains consist mostly of internal casls, 

 much distorted, with a portion of the epidermis usually preserved, and 

 occasionally a small amount of the calcareous material. The cavities left 

 by the decomposition of the teeth are sometimes quite charactei'istic, but 

 more frequently compressed and distorted so as to be of but little service 

 in comparison. The valves have usually Ijeen preserved in contact, so 

 that the hinge features do not show on tlie cast, although the muscular im- 

 prints, and often the pallial line, are very handsomely preserved. But the 

 amount of comjjression which the specimens have undergone can only be 

 judged of by the form of examples which have been imbedded vertically or 

 obliquely in the clays. This has often been so great as to place them entirely 

 beyond the possibility of determination. Owing to the differences of form 

 and the difficulty of accurate comparison with living forms, I have preferred 

 to leave them standing as distinct species, under the names originally used 

 by Mr. Lea, and have added two others from the collection of Professor 

 Lewis, following the same sy^stem of nomenclature. 



UNIONID^. 



Genus UNIO Eetzius. 



Unio nasutoides. 

 Plate XXXIV, Figs. 4 and 5. 

 Unio nasutoides Lea. Proc. A. N. Sci., Phil., 1868, p. 163. Pamphlet, 1868, p. 30. 



Shell very transverse, about twice and a half as long as high, rounding 

 from below anteriorly and pointed behind, with the beaks within the 



