150 PALEONTOLOGY OF NEW-TORK. 



189. 1. NUCULA LEV ATA ( n. sp.). 

 Pl. XXXIV. Fig3. 1 a-i. 

 Compare JVucula Itpvis, Sowerby in Sil. System, pag. 335, pl. 22, fig. 1. 



Somewhat obliquely ovate or subrhomboidal, gibbous ; anterior extremity broad, rounded ; 

 posterior side narrower and somewhat obliquely truncated ; umbones usually about one 

 third of the length from the anterior extremity, elevated and incurved ; cardinal line 

 slightly curved ; crenulations extending nearly twice as far on the posterior as on the 

 anterior side ; surface obscurely marked with concentric lines ; muscular impressions visible 

 near the anterior and posterior extremities. 



This little shell presents considerable variation in form, even in the same locality, 

 becoming greatly increased in height over the prevailing form. It often becomes almost 

 cylindrical from compression vertically ; and again, from lateral pressure, the umbones are 

 much elevated, and the shell broader than the natural form. 



This is apparently identical with the species occurring in several western localities ; 

 though in these situations it rarely or never attains the great elevation of the innbones, or 

 general gibbosity of the shell, which the eastern forms present. Notwithstanding this 

 prevailing difference, I am unable to find any characters indicating a specific distinction, 

 and therefore arrange them both under the same name. 



Fig 1 a, b. Left valve and posterior view of a specimen of the prevailing form in New- York. The 



muscular impressions t t are distinctly visible. 

 Fig. I c, d. Similar views of a larger shell, slightly varying in its proportions. 

 Fig. 1 e Left valve of a very gibbous specimen, in which the posterior side is less extended. 

 Fig. 1 / A specimen less elevated, and approaching the western forms. 

 Fig. 1 g. Cast of a western specimen, showing more distinctly the muscular impressions f t- 

 Fig. 1 k. Cardinal view of the same, sho%ving the crenulations of the hinge, and the same enlarged. 

 Fig. 1 i. A smaller specimen of the same, with the shell showing faint concentric lines. 



This is the prevailing form of the western specimens, while 1 ^ is intermediate to this 



and the common New- York forms. 



Position and locality. This shell occurs in New-York, in the central portions of the 

 Trenton limestone, at Middleville, Jacksonburgh, Trenton Falls, and other places. The 

 western specimens figured are from Mineral Point ( Wisconsin) , whence they were sent 

 with other fossils peculiar to the Trenton limestone. {State Collection.) 



