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Genus Platyceras, Conrad. 

 Platyceras paralium (n. sp.) 



Shell rather below the medium size, composed of but little more 

 than one loosely-coiled volution. Apex minute, laterally compressed ; 

 the upper half of the shell somewhat angular on the dorsum, more 

 rapidly expanding and less angular in the outer part. Body of the 

 shell marked by several proportionally strong, irregular plications, 

 which give a deeply undulating or dentate character to the margin of 

 the aperture. General form of the aperture irregular ovate. Peris- 

 tome much prolonged on the anterior portion, and a little more 

 expanded on the right side. 



Surface marked by strong, concentric lamellose lines of growth, 

 which are strongly undulated as they cross the plications. 



This small shell is of the t^-pe of P. (Capulus) acutirostre Hall, 

 (Iowa Kept., Vol. i., Part ii., pi. 23, fig. 14), being of that class which 

 have their volutions coiled on the same plane. It is easily distin- 

 guished by its laterally compressed beak, and the deep longitudinal 

 plications. 



Platyceras bivolve (n. sp.) 



Shell small, ventricose, composed of about two closely-coiled, 

 rounded volutions, spire not elevated above the surface of the outer 

 volutions. Inner whorl minute, outer volution more rapidly expand- 

 ing and ventricose. Section of the volution transversely ovate, nar- 

 rowest at the inner or ventral margin ; border of the aperture with a 

 shallow sinus on the upper side, and another below the middle. 



Surface marked by fine transverse strias, parallel to the border of 

 the aperture. 



Shell of the t}'pe of P. ventricosum, Conrad, and closely resembling 

 young specimens of that species. 



Genus Pleurotomaria, Defrance. 

 Pleurotomaria mississippiensis (n. sp.) 

 Shell rather above a medium size, spire elevated, composed of five 

 to six volutions ; the height a little greater than the diameter of the 

 base. Volutions flattened on the upper side, the plane extending 

 from the suture to the middle of the whorl, regularly rounded on the 

 inner side.^ Periphery marked by a revolving band, which on the 

 outer volution is an eighth of an inch in breadth, prominent at the 

 margins, and depressed in the centre. Volutions coiled upon each 

 other at the base of the band. Angle of the spire seventy to eighty 

 degrees. Surface characters unknown. The nature of the imbed- 

 ding material is such that it has entirely destroyed the surface mark- 

 ings; but the form of the shell is so entirely distinct from any other 

 described from rocks of the same age, that it is easDy recognized. 



