INVERTEBRATE PALEONTOLOGY. DO 



simple i)allial line, and general form. They also differ from the latter by 

 having an internal cartilage-pit. 



Until the hinge and interior of a number of Palaeozoic species that have 

 been referred to Nucula can l>e more clearly seen than has yet been done, 

 tin' exact period at which the genus was first introduced cannot be very 

 satisfactorily determined. Most of the Palaeozoic species that have been 

 described under the name Nucula belong to Tellinomya and other allied 

 groups. Some, however, seem to be true nuculas, even from the Devonian, 

 and possibly a few from the Silurian. From the Carboniferous, we have 

 species (such as N parva, McChesney) that seem to present all the charac- 

 ters of the genus, both internal and external. The genus Nucula also occurs 

 through all of the later formations, and is well represented in our existing 

 seas. It was quite abundantly represented during the Cretaceous and Ter- 

 tiary epochs, in the latter of which it appears to have attained its greatest 

 development. The recent species, like the extinct, are widely distributed, 

 the former being found generally in rather deep water on muddy bottoms of 

 the North American seas, as well as on the coasts of Japan, China, Norway, 

 Chili, &c. 



Nucula snbplaua, M. & H. 



Plato 17, ligs. 7, a, h. 

 Nucula subplana, Meek and Haydeu (Ap., 1856), Proceed. Acad. Nat. Sei. Pbilad., VIII, 85. 



Shell small, broad-ovate, approaching subtrigonal, much compressed, 

 with height about four-fifths the length ; posterior or shorter side obliquely 

 truncated above, and narrowly rounded below ; anterior side longer and more 

 broadly rounded ; basal margin semi-ovate in outline, not crenulate within ; 

 dorsum declining a little with a slightly convex curve from the beaks ante- 

 riorly, and a little concave and sloping abruptly behind ; beaks moderately 

 prominent, compressed, and located about half-way between the middle and 

 the posterior or shorter side ; teeth of the hinge about fifteen in each valve 

 in front of the beaks, and a smaller number behind ; muscular and pallial 

 impressions faintly marked. (Surface unknown.) 



Length, 0.37 inch ; height, 0.26 inch ; breadth, 0.12 inch.* 

 Having only seen internal casts of this little Nucula, I of course know 

 nothing of its surface-markings, though it was probably merely marked by 



* Fig. 7 a represents the uiubonal region too much depressed. 



