1856.] 273 



extensiTG collections, however, may hereafter prove them to be distinct species. 

 This shell may be readily distinguished from a species described by us in the 

 Proceedings of the Academy in April last, under the name of C. Deweyi, (page 

 83,) by its extreme thinness ; and from Venus? circularis of this paper, by its less 

 elevated and more obtuse beaks, as well as by the widely different form and 

 direction of the sinus of the pallial impression. It is more Mable to be confounded 

 with C. tenuis of Hall and Meek, {Mem. Am. Acad. Arts andSci.p. ^Q^, plate I, fig. 8,) 

 but presents in some of its varieties a more oval or ovate form, and is always 

 less broadly rounded at the extremities. In addition to these diiferences, it 

 comes from a distant locality, and from a higher bed, in which not a single 

 species known to occur in that in which C. tenuis is found, has yet been recog- 

 nized. This and the following species are referred to the genus Cytherea, chiefly 

 from external characters, as we have not seen the hinge of either. 



Locality and position. Two hundred miles above the mouth of Milk River, No. 

 4 of the series. 



Cytherea Owenana. 



Shell round oval, compressed ; beaks moderately elevated, placed about half 

 way between the center and the anterior end ; cardinal border convex behind 

 and concave before the beaks ; extremities rounded, the anal end being broader 

 than the buccal ; base semioval ;] anterior muscular impression narrow ovate, 

 and the posterior round ovate, both pointed above and very shallow ; sinus of 

 the pallial impression narrow and deep, directed obliquely upwards towards a 

 point a little in advance of the beaks. Length 1-56 inch; height 1-35 inch ; 

 breadth about -66 inch. 



The surface markings of our specimen are almost entirely carried away by 

 the exfoliation of the outer laminre of the shell. A small remaining portion near 

 the lower border is marked with small concentric wrinkles and much finer lines 

 of growth. We have not had an opportunity of examining the hinge. 



The greater thickness of the substance of the shell will at once distinguish 

 this species from all the allied Nebraska forms known to us, excepting our C. 

 Deweyi (see Proceedings of the Academy, April last, page 83) and C. orbiculata 

 of Hall and Meek, (Mem. Am. Acad. Arts and Sciences, page 382.) From the 

 former it may be known by its much deeper and relatively narrower pallial 

 sinus ; that of C. Deweyi being in the form of an equilateral triangle, and not 

 reaching the center, while in this it is about one-third deeper than wide, and 

 extends beyond the middle of the shell. They are, moreover, from distant locali- 

 ties, and widely separated stratigraphical positions. From the latter it will be 

 distinguished by its more oval and compressed form and less elevated beaks. 

 We name it after Dr, David Dale Owen, of New Harmony, Indiana. 



Locality and position. Mouth of Judith River, from a sandstone supposed to 

 be the same as No. 1 of the series. 



LUCINA OCCIDENTALIS. 



? Tellina occidentalis (Morton.) Jour. Acad. Nat. Sci. vol. 8, pi. si., fig. 3. 



Mould of Lucina? (Owen.) Report Wis., Iowa and Min. tab. vii., fig. 8. 



Shell rather thick, transversely oval, compressed ; cardinal border slightly 

 concave, and sloping very little in front of the beaks, convex and declining 

 behind ; anal end vertically subtruncate at the extremity, and having sometimes 

 a very obscure ridge passing obliquely backwards and downwards from the 

 beaks ; anterior end rounded and a little wider than the posterior ; base semi- 

 ovate, more prominent before than behind the middle ; beaks small approximate, 

 little elevated above the hinge, slightly in advance of the center ; lunule very 

 small, lanceolate ; surface ornamented by distinct irregular concentric lines. 

 Length (large specimen) 1'90 inchj height 1*56 inch ; breadth -94 inch. 



By breaking open some of the specimens, we find under the beaks two di- 

 verging cardinal teeth in each valve ; and in front of these one anterior lateral 

 tooth in the right valve, which fits between two smaller ones in the left. If 



