GENUS RENSSELiERIA. 
385 
CJMUS ReNSSELJGRIA* (Hall). 
The following species possesses many of the important characteristics 
of the Genus Renssel^iria, in the elongate-oval or subcylindrical form. 
The punctate shell and the muscular impression of the dorsal valve 
correspond with shells of that genus. 
Reiisselseria ? johanni (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 
sloping 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- 
SELiERiA ; while 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 are 
only moderately convex, and there is an indication of area upon the dorsal valve. 
' The materials 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 
examination of the internal structure. The form of the shell, incurvation of the ventral 
valve, and general aspect are very similar to RENSSELiERIA ; 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 Rensselandia. 
Geological formation and locality. In limestone of the age of the Upper Helderberg, 
Waterloo, Iowa ; from Mr. 0. St. John. 
# RexsselvEria, Hall : proposed and printed in Vol. iii, Palaeontology of New-York, 1858; pub¬ 
lished in Twelfth Report on the State Cabinet, p. 39. 1859. 
4, [ Paleontology IV.] 49 
