238 PALEONTOLOGY OF NEW YORK. 



The same form of shell occurs rarely in Wisconsin (Door county), and has been 

 described by McCuesnky as P. bisintuittis* 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 fauna 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 subredus. 



With all these variations in exterior there are some slight differences in the 

 interior structure. A concave deltidium is sometimes retained, and a faint 



Fig. 1C9. FlO. 170. Fic. 171. Flu. 172. 



Fig. 169. /•en<nmeru» oiton^tM, Sowerby. A tiaiiBVcrsc section, showing llio septa. (C.) 



Figs. 170, 171. Transverse sections of the septa of Pentamenu oblongut. Fig. 170 shows the septum of the pedicle- 

 valve and the enclosure of its bate by the shcllsubstance of the valve. i"ig. 171 is an enlargement of 

 the septa of the brachial valve, and shows a thin coating of testaceous matter upon the inner faces of 

 the jirismatic walls. (c.) 



Tig. m. Pentamenu cylindriciu, UaW, 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 



•Desci-i 1)1 ions of New Sjiecies of Fossils, p. 8.5, j)!. ix, fig. 1. 1859. 



