GENUS RENSSEL.ERIA. 385 



GEXrS ReXSSELJIRIA* (Hall). 



The following species possesses many of the important characteristics 

 of the Genus Renssel.eria, in the elongate-oval or subcylindrical form. 

 The punctate shell and the muscular impression of the dorsal valve 

 correspond with shells of that genus. 



Rensselaeria ? jolianni ( n. s). 



PLATE LX. 

 Shell oval or subovate, and, in full-grown individuals, elongate oval in 



outline, or subcylindrical : valves subequally convex. 

 Ventral valve, in young specimens, more gibbous in the upper part, and 



eloping to the front. In older shells the valve is regularly convex, 



sometimes prominent or » subangular along the middle; the beak 



closely incurved over that of the opposite valve. 

 Dorsal valve regularly convex ; in old shells, acquiring nearly the same 



convexity as the ventral valve : in some specimens, much, less con- 

 vex, and extended on the hinge-line. 

 Surface marked by concentric striae of growth, which are at intervals 



crowded into stronger ridges or imbricating folds. Shell-structure 



punctate. 



The cast of the dorsal valve shows muscular imprints similar to those of Rens- 

 SEL.«KiA ; wbHe in the cast of the ventral valve, the filling of the rostral cavity 

 is less prominent. 



In some young specimens, which appear to be of this species, the valves arc 

 only moderately convex, and there is an indication of area upon the dorsal valve. 

 The materiabi in my possession are in such a condition as to prevent a satisfactory exami- 

 nation of all the parts, and a positive reference of the species can only be made from an 

 examiuaticn of the internal structure. The form of the shell, incurvation of the ventral 

 valve, and general a>pect are very similar to Renssel^ria ; but the characteristic radia- 

 ting striae have not been observed upon the surface. 



Should its interior structure prove it generically distinct from that genus, I would propose 

 the name Renssklandia. 



Geological formation aiid locality. In limestone of the age of the tapper Helderberg, 

 Waterloo, Iowa ; from Mr. 0. St. John. 



• Rensskl.kria, IIai.l : proposed and printed in Vol. iii, Palajontology of New- York, 1858; pub- 

 lished in Twelfth Rei)ort on the State Cabinet, p.. 39. 1859. 

 [PAL.E0NT0I.0aif IV. J 49 



