366 Systematic Paleontology 



In the Keyser member, especially in the ballast quarries near Keyser, 

 are found shells that are best referred to [/. nudeolatus. These are always 

 smaller than the Xew Scotland individuals of New York, have somewhat 

 fewer plications (eighteen to twenty against eighteen to twenty-three in 

 northern specimens), and are never so inflated or strongly triangular in 

 section. These differences are not considered of enough importance for 

 distinction under another name, and the shells are here referred to U. 

 nucleolatm (see also the remarks on U. l-etjserensis) . The geologic range 

 is therefore extended from the Keyser well up in the Xew Scotland, being 

 abundant and restricted to the former horizon in the South, but attaining 

 the climax of development in variation and number in the New Scotland 

 of the Helderberg ^Mountains. 



Length and width about 1.5 cm. 



Occurrence. — Helderberg Formation, Keyser Member. Keyser, 

 West Virginia. Coeymans Member. Corriganville, Devil's Backbone, 

 Tonoloway. 



Collection. — U. S. National Museum. 



TJncinulus nucleolatus var. angulatus n. var. 

 I'late LXIV, Figs. 18-30 

 Dcscripiion. — Shell subpentagonal in outline, width more than two- 

 thirds the length. Ventral valve convex, not at all gibbous, approaching 

 more to plano-convex ; beak sharp, pointed, but slightly incurved and ex- 

 tending beyond that of the dorsal valve. Dorsal valve convex, larger than 

 the ventral valve; beak slightly incurved beneath that of the ventral 

 valve, lateral margins abruptly deflected. Both valves have about equal 

 convexity and are slightly deflected toward the lateral margins, the dorsal 

 valve being more convex is more abruptly deflected toward the lateral 

 margins ; both valves gently curved in front. Surface marked by 23 well- 

 developed simple plications in addition to which there are several indistinct 

 plications near the lateral margins. The plications are very prominent 

 from the center towards the front, becoming obsolete towards the beak. 

 Four plications form a slight elevation on the dorsal valve and three 

 occupy a shallow sinus on the ventral valve. The sinus is produced in 



