238 
PALEONTOLOGY OF NEW YORK. 
The same form of shell occurs rarely in Wisconsin (Door county), and has been 
described by McChesney as P. bisinuatus* a name which may serve a useful pur¬ 
pose as a varietal designation. About Richmond, Indiana, a broader, more 
ovate shell predominates, which does not widely differ from the characteristic 
form of the Clinton hiuna of New York. At Utica, in the same State, and in 
the vicinity of Louisville, the narrow elongate shell, P. oblongus, var. cylindricus, 
abounds ; it is usually deep-valved and distinctly trilobed. 
Among the shells occurring in the dolomites of Wisconsin there is a great var¬ 
iation in form, with a tendency to increasing depth of valves, but these variations 
are less extreme, and their geographic value has not been determined. Thus 
also with the representatives of the species in the dolomites of Iowa (Earlville 
and elsewhere). In the siliceous beds of the Niagara group in the latter State 
(Jones county), there is a small, ovate, often elongate variety, with the triloba- 
tion rather faintly marked, and a quite distinct form in the rusty chert of the 
same county, the latter a subquadrate shell, very broad across the cardinal 
region, with nearly straight, parallel lateral margins, very full and prominent 
umbo, distinctly trilobate surface, the median lobe being divided by a linear 
axial groove on both valves. This is so well defined a shell and so distinctively 
local in its value that it may receive the varietal designation subrectus. 
With all these variations in exterior there are some slight differences in the 
interior structure. A concave deltidiurn is sometimes retained, and a faint 
Fia, 170. Fig. 171. 
Fig. Pentamerus ohlongus,&owexhy. A transverse section, showing the septa. (C.) 
Figs. 170, 171. Transverse sections of the sejita of Pentamerus oblongus. Fig. 170 shows the septum of the pedicle- 
valve and the enclosure of its base by the shell-substance of the valve. Fig. 171 is an enlargement of 
the sepia of the brachial valve, and shows a thin coating of testaceous matter upon the inner faces of 
the prismatic walls. (C.) 
Fig. 172. Pentamerus cylindrictis, Hall. A transverse section, showing the septa. (c.) 
lobation of the apical end of the spondylium is the sole evidence of a cardinal 
process. The depth of the spondylium and septa varies with the convexity of 
* Descriptions of New Species of Fossils, p. 85, pi. ix, fig-. 1. 1859. 
