104 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



distinguished by the shell alone, which is usually more attenuated posteriorly, 

 with more angular posterior umbonal ridges; but more particularly by its very 

 slightly sinuous pallial line. It also more frequently has its beaks directed a 

 little backward. There are species, however, that closely resemble Yoldia 

 in most of these characters, excepting in not having the pallial line so deeply 

 sinuous as in that genus. 



From Nucula, with which it was formerly confounded, Nuculana is 

 readily distinguished by its general form, less angular hinge, pallial sinus, and 

 less pearly shell 



The genus Nuculana is known to range back at least to the Carboniferous; 

 several species being known from the Coal-Measures, showing all of the external 

 and internal characters of the group. Others, presenting at least all of the 

 external characters, occur in the Lower Carboniferous, and a few in the De- 

 vonian. Species have also been referred to it (under the name Leda) from 

 Silurian rocks, but these probably belong to Tellinomya, Nuculites, and other 

 distinct genera. The Permian species N. vinta (— Leda vinta, King), has 

 its pallial line nearly or quite simple, and has been, for that reason, referred 

 by Professor McCoy to the genus Nucula, but it has the form and habit of 

 Nuculana, and, even if not belonging to the latter, could hardly be properly 

 included in the genus Nucula. 



This genus also ranges through the Triassic and all succeeding forma- 

 tions, and probably attains its greatest development in the seas of the present 

 epoch ; where it has a wide geographical distribution, being found on the coasts 

 of Siberia, Great Britain, Japan, Australia, the West Indies, &c. 



Nuculana bisulcata, M. &H. 



Plate 15, fi;;s. 4, a, b. 



.Leda bisulcata, Meek and Hayden ( 18(51), Proceed. Acad. Nat. Sci. Philad., XIII, 440. 

 Nuculana bisulcata, Meek (1864), Smithsonian Check-List. Cret. Foss. N. Am., 8. 



Shell small, transversely subovate, rather gibbous in the central region ; 

 anterior side narrowly rounded or subangular above the middle ; base nearly 

 semi-elliptic, with a very slight sinuosity near the posterior extremity ; 

 posterior end compressed, narrow, and subangular in outline above the middle; 

 dorsal border sloping from the beaks at an angle of about 135° ; beaks rather 

 obtuse, placed slightly in advance of the middle; posterior umbonal slopes 

 distinctly angular, the angles extending from the beaks along very near the 



