44 PALEONTOLOGY OF THE UPPER MISSOURI. 



to a lino drawn from the beaks across to the most elevated part of the posterior extremity, 1.50 inch ; greatest 

 convexity near the middle, 1.11 inch ; breadth of posterior hiatus, 1.07 inch ; height of do., 1.44 inch. 



This species is so nearly like Mya dilatata, Phillips (as figured by Morris & Lyatt, 

 Moll. Gt. Ool. vol. X, fig. 5), from the English Oolite, that if found imbedded 

 with it in the same rock, they might, on a hasty examination, be mistaken for 

 varieties of the same shell. On comparison, however, it will be seen that the 

 Kansas species has its beaks more elevated, and placed nearer the anterior end ; it 

 is also a little broader shell, in proportion to its length, and rather straighter on the 

 dorsal and ventral margins. 



Locality and position. Near the level of the Missouri River, at Leavenworth 

 City, Kansas. Coal Measures. (Type 1019.) 



Chaeiiomya Cooperi. 



(PLATE II, Fig. 2, a, b.~) 



Panopma Cooperi, MEEK & HATDEX, Trans. Albany Inst. IV, March 2d, 1858, 11. 

 Allorisma ? Cooperi, MEEK & HAYDEX, Proceed. Acad. Nat. Sci. Phila. Dec. 1858, 264. 



Shell oblong subcylindrical ; posterior side very abruptly truncated, a little oblique, and widely gaping; anterior 

 side more compressed, and rather narrowly rounded. Base nearly straight along the middle and behind, rounding 

 up in front ; dorsal side concave, and not declining behind the beaks, sloping rather abruptly in front. Heaks 

 depressed, incurved, contiguous, and located about half way between the middle and the anterior end. Inflected 

 cardinal margins a little thickened within, so as to leave a deep narrow depression along the cardinal border, in 

 casts of the interior. Surface (of internal casts) marked by small, rather obscure concentric ridges, which are 

 most distinct and regular on the umbones, and gradually fade away on other parts of the shell. (Muscular and 

 pallial impressions unknown.) 



Length, 2.57 inches ; height, 1.37 inch ; convexity or breadth, 1.12 inch ; breadth of posterior hiatus, 1 inch. 



This species will be readily distinguished from the last, by its proportionally 

 shorter form, and more distinctly truncated and shorter posterior side. Its beaks 

 are also rather less prominent, and marked by finer and more distinct concentric 

 ridges. It seems to be more nearly related to a species described from the Coal 

 Measures of Kansas, by Prof. Swallow, under the name of Allorisma ? minneJiaJia, 

 though our specimen wants the oblique ridge mentioned in the description of that 

 species. Should they prove identical, however, the specific name Cooperi will have 

 to take precedence, as it was published nearly a month in advance of the issue 

 of Prof. Swallow's description. 



Locality and position. Helena, Kansas Territory. Coal Measures. (Type 1018.) 



CLASS GASTEROPODA. 



SUB-CLASS PROSOBRANCHIATA. 

 ORDER Riphidoglossata* 



SUB-ORDER PODOPTHALMA. 



FAMILY PLEUROTOMARIID^E. 



Shell varying in form, thickness and ornamentation, according to the 

 genera and species ; imperforate or more or less widely umbilicate, pearly 

 within. Aperture not sinuous, or produced below. Outer lip with a 



