INVERTEBRATE PALEONTOLOGY. 1('»:i 



really have constantly bu1 two cardinal teeth to each valve, as the figures 

 seem to indicate, and as staled by the authors who described some of them, 

 they will probably be found to belong mainly to one or more distinct groups. 



In the Tertiary rocks, however, typical tonus of the genus are certainly 

 found; and it seems to have reached its maximum development in the 

 muddy bottoms of the rivers and estuaries of the present time. The existing 

 species are widely distributed in America, India, China, Africa, &c. 



The types of the groups Leptesthes and Veloritina, as already stated, 

 seem to come from near the junction of the latest Cretaceous and the oldest 

 Tertiary. Although these groups appear to have no closely-allied representa- 

 tives among living species, they certainly agree quite closely with European 

 Eocene forms. 



Corbie u la! nncalis, Meek. 



Plate 1, fig. 5, 11. c. 



( 'orbicula .' nucalis, Meek (1872), Haydeu's Second Report Geol. Survey of the Territories, 304. 



Shell small, trigonoid-suborbicular, gibbous, the greatest convexity being 

 a little above the middle; basal margin forming nearly a semi-elliptic curve; 

 posterior subtruncated or rounded ; anterior margin rather narrowly rounded, 

 its most prominent part being below the middle; dorsal outline sloping rather 

 abruptly, and nearly equally in both directions, with slight convexity of out- 

 line near the beaks behind, and about the same concavity in front; beaks 

 nearly or quite central, incurved, with slight forward obliquity; posterior 

 dorsal surface sometimes very slightly furrowed immediately behind (lie 

 umbonal slope in internal casts; muscular impressions shallow, comparatively 

 rather large, and arcuate-subovate in form ; pallia! line with a shallow, obtuse 

 sinus. External surface unknown ; that of cast smooth. 



Length, 0.47 inch; height, 0.42 inch; convexity, 0.26 inch. 



Impressions of the hinge of this shell left in the rock, show that it has, 

 in the right valve, long, narrow, transversely-striated, double lateral teeth, as 

 in Gorbicula. The specimens of the impressions of the cardinal teeth are 

 not so clearly-seen, beinn mainly hidden behind the cast of the umbones in 

 the only specimen that I have seen with this part of the hinge well preserved. 



far as is yet known, in regard to the geological range of the genus in America. We at this time only 

 know a few species from the Cretaceous of this country probably belonging to this genus, and 

 these came from the upper portions of the system, t am not aware that any rocks hi).ve been, up to this 

 time, certainly determined to belong to the age of the Wealden in America. 



