Weller — Kinderhook Faunal Studies. 449 



margin which is very short and sharply rounded ; ventral mar- 

 gin slightly convex posteriorly, becoming gradually more 

 curved anteriorly where it passes into the regularly rounded 

 anterior margin. Valves rather strongly convex anteriorly, 

 the umbo prominent, becoming gradually more depressed pos- 

 teriorly, the umbonal ridge subangular, following the postero- 

 cardiual margin of the shell, the cardinal slope from the ridge 

 nearly vertical, in larger specimens even undercut so that in 

 a direct view of the valve the slope cannot be seen. Surface 

 marked with very fine, regular, concentric lines, about five or 

 six occupying the space of one millemeter. 



The dimensions of two specimens, one right and one left 

 valve, are: length 16 and 10.5 mm., width 9 and 6.2 mm., 

 convexity 3.75 and 3 mm. 



Remarks. This little shell is a close ally of N. diversa Hall, 

 from the Hamilton fauna of New York, but it may be dis- 

 tinguished from that species by the more abrupt cardinal slope 

 from the umbonal ridge, in the Hamilton shell the surface 

 of the slope being clearly visible in a direct view of the valve, 

 while in the Glen Park species it can only be seen in a cardi- 

 nal view. In the Glen Park species also, the posterior ex- 

 tremity of the shell is less angular than in the Hamilton 

 species. This species is an exception to most of those in the 

 Glen Park fauna with close relatives in the Hamilton fauna, 

 in that it is larger than its Hamilton ally, 11 mm. being the 

 longest specimen of N. diversa recorded by Hall, while the 

 Glen Park species attains a length of at least 16 mm., although 

 about 10 mm. is the usual length. 



Several species similar to JST. dtversoides have been de- 

 scribed by Herrick from the Waverly formations of Ohio. 

 Of these JV. spatulaia has a more sharply angular posterior 

 extremity than ISF. diversoides, and W. similis has a much 

 more strongly curved ventral margin. The same author has 

 identified other similar shells with iV". diversa Hall. JST. sac- 

 cata (Win.), is another closely related species originally de- 

 scribed from the higher " yellow sandstone " at Burlington, 

 Iowa, and later identified by Herrick in the Waverly ; this 

 species seems to be more elongate than the Glen Park speci- 



