GIRTY, THE WEWOKA FORMATION OF OKLAHOMA 133 



less symmetrical, somewhat truncated at the narrow extremity. Greatest 

 height about one third tlie shell length in front of the posterior margin. Con- 

 vexity rather high, chiefly situated in the posterior third of the shell ; com- 

 pressed anteriorly, somewhat compressed near the posterior margin as well. 

 Beak small, strongly incurved, pointing toward the short end of the shell. 



Surface not well shown, often apparently smooth. A few examples are 

 marked by regular, closely arranged concentric striae, and many show more 

 prominent irregularities of growth. 



The dentition consists of about six large posterior teeth and about 30 small 

 anterior teeth. An interval beneath the beak is probably occupied by a num- 

 ber of additional teeth of small size. 



There is the usual complement of large muscle scars, one anterior and one 

 posterior, situated near the dorsal border. Apparently a small elongate scar 

 occurs between the anterior adductor and the beak, close to the cardinal line. 

 Possibly a corresponding scar occurs also between the beak and the posterior 

 adductor. This arrangement is. therefore, very much as I have observed it in 

 Leda bellistriata, but of the curved oblique internal ridge in the umbonal 

 region with its attachment (?) scar, no equivalent structure has been observed 

 in the present form. The shell is considerably thickened between the beak and 

 the large posterior adductor, however, causing a deep excavation in internal 

 molds, sharply defining the anterior boundary of the scar. The pallial line 

 appears to be entire, or with a faint deflection in the anterior portion. It has 

 not been clearly seen in that portion. 



Horizon and locality : Wewoka formation ; Wewoka quadrangle, Coal- 

 gate quadrangle, Okla. 



Nuculopsis gen. now 



The present genus is based upon Nncula ventricosa of Hall and it is distin- 

 guished from Nucula primarily because, though the beaks point toward the 

 short side of the shell, that side is not posterior but anterior. The determina- 

 tion of this fact rests upon the occurrence of a ligamental groove or area along 

 the hinge margin on the long side of the shell. Though varying in the dis- 

 tinctness with which it is shown, traces of this structure can be observed in 

 nearly all the lai'ge number of specimens examined. It is. therefore, a real 

 and persistent character of the species and is hardly open to any other inter- 

 pretation than that which I have put upon it. The existence of this structure 

 then, which has no homologue in Nucula, and its almost definitive importance 

 in determining the long side instead of the short side of the shell as posterior, 

 constitute the most important differences from that genus. 



The shape is elliptical rather than triangular and the long side is rather 

 produced for Nuciila itself. The beaks are conspicuously turned toward the 

 shorter side. The typical species has a distinct, though ill-defined and narrow, 

 constriction near the anterior extremity. The lunule and escutcheon are poorly 

 defined. The surface is generally almost smooth. On the interior, there are 

 the usual large posterior and anterior adductors, in addition to which, between 

 those scars and the beaks, a third and fourth pair of muscular imprints can be 

 seen. The dentition consists of a continuous series of taxodont denticles not 



