1856.] 85 



smooth ; teeth of the hingje numerous, short, obtuse, closely interlocking. Length 

 64 inch ; breadth -21 inch ; height -25 inch. 



Even casts of this species maj- always be known from the last, by the central 

 position of the beaks and more narrowly rounded extremities. 



Locality and position. Same as last. 



Nucni-A SUBPLANA. Shell small, oral, compressed ; anterior end obliquely 

 subtruncate from the beaks a little more than half way down; posterior ond 

 round, base broadly rounded ; beaks prominent, compressed, located about half 

 way between the centre and the anterior end ; surface (of cast) having a few 

 faint indications of concentric undulations ; teeth of hinge moderately long ; 

 border smooth. Length -25 inch ; breadth -09 inch ; height -20 inch. 



Of this little Nucula we have only seen casts. It may be known from other 

 species from these formations by its short compressed form and elevated beaks. 

 It is rare. 



Locality and position. Yellow-stone River, one hundred and fifty miles above 

 mouth, in a bed supposed to be upper part of No. 4 of series. 



NuccLA CANCELLATA. Shell rather large and thick, ovate or subtriangular, 

 ventricose, pearly within ; anterior end short, obliquely truncate, posterior end 

 longer, narrowly rounded ; umbonal region gibbous ; beaks somewhat elevated, 

 slightly incurved ; escutcheon lanceolate ; lunule ovate, flattened, but scarcely 

 impressed ; surface ornamented by numerous flatlj^ rounded, simple, closely ar- 

 ranged, radiating costs, which are crossed by small, irregular, concentric 

 wrinkles, and finer indistinct lines of growth ; border neatly crenulated. Length 

 93 inch ; breadth -50 inch ; height -65 inch 



This beautiful Nucula resembles in its surface markings N. pectenata of Sow- 

 erby, (Min. Conch, vol. 2, page 209, fig. 6, V,) but differs in the form and depth 

 of its lunule, which is rather narrow ovate and ver}' shallow, while in Sowerby's 

 species it is distinctly cordate and deeply impressed. The muscular impressions, 

 though large, are much more shallow than in N. pectenata. The radiating costee of 

 our species become obsolete on the lunule and escutcheon, and are about five 

 times as broad as the grooves between, near the border. The anterior muscular 

 impression is bordered by a distinct ridge, which extends nearly up to the beaks. ' 



Locality and position. Moreau River, No. 5 of the series. 



Nucula planomarginata. Shell ovate, somewhat compressed, pearly within ; 

 anterior end very short, obliquely truncate ; posterior end long and narrowly 

 rounded, dorsal and ventral margins forming, from the beaks backwards, ellip- 

 tical curves ; beaks much elevated ; surface unknown ; muscular impressions 

 faint ; border smooth. Length -95 inch ; height GO inch ; breadth -26 inch. 



We have only seen internal casts of this species, the shell being always left 

 adhering to the matrix, from which we infer the surface was ornately marked. 

 It may be at once distinguished from the last by its more compressed form and 

 smooth border. 



Locality and position. Same as last. 



Pectcnculina parvula. Shell very small, obliquely oval, somewhat com- 

 pressed, inequilateral ; buccal end and base rounded ; anal extremity rounded 

 and slightly extended obliquely downwards ; cardinal border short, and having 

 a narrow, well defined, longitudinally striate area on each valve ; beaks small, 

 not much elevated, about midway between the centre and the anterior end ; 

 surface polished and marked with lines of growth, sometimes crossed by faint 

 indications of radiating costae ; hinge having in each valve three or four teeth 

 on each side of the central triangular ligamentary pit ; interior marked with 

 faint radiating grooves terminating in distinct crenulations at the border. Length 

 18 inch ; breadth -10 inch ; height IS inch. 



This little shell might, upon a hasty examination, be mistaken for Cardium 

 varum, (Evans and Shumards unpublished MS.), but it possesses all the cha- 

 racters of the above genus. The shell is often so translucent that the internal 

 radiating grooves are seen through it. 



Locality and position. Yellow-stone River, No. 5 of the series. 



