LAMELLIBRANCHIATA. 511 



CYTHERODON, Hall. 1873. 

 Cytherodon nasutus. 



PLATE LXXV, FIGS. 10-12 ; AND PLATE XCV, FIG. 30. 



CythenxUm , H.ali,. Twenty-third Ann. Reji. N. Y. State Cal). Nat. Hist. Kx. jil., pi. 14, fig-. 21. 1872. 



" {Sehiz<>dus)nasittus,ll\Lh. Pal. N. y.,v'ol. v, pt. 1. Plates and Explanations : PI. 75, figs. 10-12. 



18S3. 



Shell large, obliquely ovate or sub-rhomboid al ; length a little greater than the 

 height. Basal margin broadly rounded in the middle and anterior portion, 

 constricted near the post-inferior extremity, which is sub-nasute and abruptly 

 rounded. Posterior margin obliquely sub-truncate. Cardinal line arcuate, 

 declining posteriorly. Anterior end short, sloping very rapidly from the 

 beaks and rounded below. 



Valves convex in the lower and posterior portions, becoming gibbous in 

 the middle and umbonal region. 



Beaks at about the anterior fourth, small, attenuate, rising above the hinge- 

 line. Umbonal slope defined by a flattening of the valve or shallow furrow 

 extending from the beaks to the sinus in the basal margin. Post-cardinal 

 slope moderately convex, with the margin abruptly descending from the 

 umbonal slope to the cardinal line. 



Test thick. Surface marked by fine concentric striae which are sometimes 

 aggregated into fascicles. 



Muscular scars distinct. Inter-pallial area in the cast marked by numer- 

 ous, very small pustules. Hinge thickened, encroaching upon the cavity of 

 the beaks, which is excavate in the cast of the interior. Cardinal teeth inter- 

 locking, situated beneath and anterior to the beaks. There are apparently 

 four cardinal teeth in the right valve and five in the left. 



The specimen described is a cast of the interior, and imperfect along the 

 basal margin. When entire it has measured about 53 mm. in length and 

 about 50 mm. in height. 



In its general form this fossil somewhat resembles several species of Schizodus, 



