6io CRETACEOUS PALEONTOLOGY. 



Description. — The dimensions of a very perfect left valve are : 

 heig^ht, 1 6 mm.; length, 19 mm.; convexity, 4 mm. Shell sub- 

 ovate in otitline; the beaks at about the anterior third, rather 

 small, directed anteriorly, scarcely incurved. Antero-cardinal 

 margin concave Just in front of the beak ; anterior, ventral, pos- 

 tero-cardinal margins convex ; the posterior margin broader than 

 the anterior. Valves regularly convex, the surface sloping more, 

 abruptly to the cardinal margins ; in front of the beaks is a nar- 

 row, scarcely impressed lunule. Hinge of the left valve with two 

 cardinal teeth diverging from beneath the beak, leaving a tri- 

 angular pit between, and a much thinner, more elongate tooth 

 directed obliquely backward close up to the ligamental area; in 

 front of the cardinal teeth is a single strong lateral tooth beneath 

 the lunule, parallel with the shell margin. Surface of the shell 

 marked by fine, concentric striae of growth, those covering the 

 area from the beak downward about 10 or 12 millimeters are 

 very regular, the interspaces gradually increasing until the 

 outer ones are about one-half millimeter apart. Beyond this regu- 

 larly marked area the lines of growth are less conspicuous and 

 not so regular. 



Remarks. — ^The specimens which have been taken as typical 

 representatives of this species are from the Marshalltown clay- 

 marl near Swedesboro, and have the shells perfectly preserved. 

 They agree closely with Whitfield's figure 10, a specimen from 

 Holmdel retaining the shell, but referred by that author to Cal- 

 Usta delawarensis. It is not certain, however, that these are 

 identical with Gabb's original type specimen, which was an 

 internal cast. No internal casts have been met with in the recent 

 collections of the Survey which seem certainly to belong to this 

 species ; in fact, it would probably be impossible to distinguish 

 between the casts of this species and those of some other members 

 of the genus. In the regular concentric markings of the area 

 about the beak, these little shells from SwedesborO' resemble small 

 individuals of Cyprimeria, but they do not possess the bent valves 

 of that genus, and the hinge characters are different. 



Formation and locality. — Marshalltown marl, near Swedes- 

 boro (177); Wenonah sand, near Crawfords Corner (126^); 



