08 [March, 



Capulcs feagilis. Shell broadly conical, very thin ; apex elevated, central ; 

 slopes slightly convex, divergence 80 ; surface marked with fine, closely arranged, 

 concentric stria3 ; aperture circular. Diameter of base 1.21i inch; height of 

 apex about .74 inch. 



It is with some doubt we refer this species to the above geaus, as our specimen 

 only consists of an internal cast with a few fragments of shell adhering ; no 

 muscular impressions are visible upon it, but the nipjile at the summit appears 

 to have occupied the interior of an attenuated and laterally curved apex. 



Locality and position. Fox Hills. No. 5 of the series. 



Genus IIELCION (Montfort). 



We have not yet had an opportunity of seeing the interior of any of the fol- 

 lowing patelliform shells, but, judging from external characters, they appear 

 referable to the above genus as defined by D'Orbigny. They are all strictly 

 symmetrical, with, as far as can be ascertained, thin, smooth, and nearly, if not 

 quite entire borders. 



Helcion sexsulcatus. Shell patelliform, longer than wide ; apex between 

 the centre and anterior margin depressed ; lateral slopes nearly straight, di- 

 vergence 100; anterior slope slightly concave, and having two broad shallow 

 grooves radiating from the apex to the antero-lateral edges ; posterior slope 

 convex, and having four broad shallow grooves radiating from the apex to the 

 posterior and lateral edges ; base or aperture oval ; surface unknown. Length 

 unknown, breadth about 1.29 inch, height .48 inch. 



Locality and position. Yellow Stone, one hundred and fifty miles from mouth. 

 No. 4 of the series. 



Helcion patelliformis. Shell patelliform, thin, a little longer than wide ; 

 apex somewhat elevated, nearer the centre than the anterior margin ; lateral 

 slopes nearl)^ straight, divergence about 90 ; anterior slope straight or slightly 

 concave, posterior convex, divergence of the two about 103 ; surface marked 

 with fine concentric lines of growth, which are crossed hj very faint indications 

 of. fine radiating striae, and on the anterior side by three or four scarcely per- 

 ceptible, radiating carinte (the latter not always present) ; aperture broad, oval. 

 Length 1 inch, breadth .84 inch, height .44 inch. 



Locality and position. Same as last. 



Helcion alveolus. Shell patelliform, thin, longer than wide ; apex depressed, 

 nearly central ; lateral slopes convex and forming an angle of 80 ; posterior 

 and anterior slopes about equally convex, divergence 120; surface marked with 

 very fine concentric lines of growth ; base or aperture elliptical or subovate, 

 Length .63 inch, breadth .43 inch, heigh th .17 inch. 



Locality and position. Yellow Stone river. No. 4 of series. 



Helcion subovatus. Shell patelliform, longer than wide ; apex depressed, 

 situated about half way between the centre and the anterior margin ; lateral 

 slopes convex, and forming an angle of 80 ; posterior slope more convex than 

 the anterior, divergence of the two 104; surface marked by faint, irregular, 

 concentric undulations and imbricating lines of growth ; base or aperture oval 

 or subovate, extremities broadly rounded, the posterior end being a little wider 

 than the anterior. Length 1.16 inches, breadth .81 inch, heighth .39 inch 



Locality and positio7i. Same as preceding. 



'! 



Helcion carinatus. Shell very obliquely conical or bonnet-shaped, thin ; 

 apex elevated near the anterior margin, and directed forAvard ; anterior and 

 antero-lateral slopes profoundly concave ; posterior slope convex above and 

 flattened or concave below. A single strong, rounded carina passes from the 

 apex down the posterior slope, becoming gradually obsolete near the margin, 

 while two or three much fainter ones radiate from the apex down the postero- 



