TRENTON CONGLOMERATE OP RYSEDORPH HILL 19 



vail, and specimens without the corrugations are only rarely 

 observed in the gray limestone, these forms seem to represent a 

 local and perhaps provincial variety and aid in indicating pro- 

 vincial differences in the Trenton faunas of New York. These 

 varietal features, specially the strong corrugations, seem not to 

 be of frequent occurrence in other places, for none are mentioned 

 in the full descriptions of L e p t a e n a sericea by Hall, 

 Sardeson, Winchell and Schuehert. The collections of the state 

 museum prove that such forms are occasionally, though rarely 

 found in the Trenton of other parts of the state; and specimens 

 found at the old Dudley observatory, Albany and at Green Island 

 give evidence that in this region it passed also into the upper 

 Utica beds. In the latter localities it is frequently intermingled 

 with typical specimens and can therefore hardly be considered a 

 different species. (Cement and group 7) 



Plectambonites pisum sp. nov. 



PI. 1, fig. 8-20 



There occurs rarely in the gray crystalline limestone, and very 



profusely in the black compact limestone, a small, extremely 



gibbous species of Pleetambonites which may be described as 



P. pisum. 



Diagnosis. Shell small, semicircular in outline, with sub- 

 auriculate cardinal extensions; highly concavo-convex, the con- 

 vexity surpassing that of a hemisphere; toward the cardinal ears 

 becoming depressed convex; length to width as 4:5, greatest 

 width along the hinge line, which is nearly straight. Surface 

 marked with very fine striae, which usually are interrupted by 

 from 16 to 20 coarse striae; sometimes the fine striae become 

 nearly obsolete, leaving the interspace between the coarse striae 

 almost smooth; at other times the coarse striae disappear, leav- 

 ing the shell uniformly and finely striated; a few concentric 

 growth lines are also present. Pedicle valve extremely gibbous, 

 the greatest elevation being in the central part; the umbonal part 

 sloping abruptly; the umbo being protuberant and projecting 

 beyond the cardinal line; anterior and lateral slopes less abrupt, 



