LAMELLIBRANCHIATA. 449 



SCHIZODUS APPRESSUS. 



PLATE LXXV, KIGS. 3-9. 



Nuculites appressa, Conrad. Jour. Acad. Nat. Sci. Phila., vol. viii, p. 248, pi. 15, tig. 4. 1842r 

 SchUodus appressus (Conrad), Hall. Prelim. Notice Lameliibranchiata, 2, p. 95. 1870. 

 Cytherodcm " " " Twenty-tliird Rep. N. Y. State Mus. Nat. Hist. Expl. pi. 14, 



fig-. 20. 1872. 

 " (Schizodus) appressus (Conrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 



Pi. 75, figs. 3-9. 1883. 

 Schizodus Cayuga, Hall. Prelim. Notice Lameliibranchiata, 2, i>. 95. 1870. 



Shell of medium or large size, sub-rhomboicl-ovate ; length one-sixth to one- 

  fourth greater than the height ; basal margin regularly rounded, with a 

 slight constriction or truncation of the margin near the posterior end. Pos- 

 terior extremity obliquely truncate. Cardinal line slightly arcuate, less than 

 half the length of the shell. Anterior end short, broadly rounded into the 

 basal margin. 



Valves moderately convex below, becoming somewhat gibbous in the mid- 

 dle and above. 



Beaks at about the anterior third or fourth, moderately prominent, rising 

 but little above the hinge. Umbonal slope sub-angular, distinct, extending 

 to the post-inferior extremity which is often somewhat nasute. 



Surface marked by fine concentric strias, which are aggregated into fasci- 

 cles on the lower and posterior parts of the shell. 



Muscular impressions and pallial line distinctly marked. Valves marked, 

 on the interior, by two narrow furrows along the umbonal slope, showing as 

 two ridges in the cast. Left valve with a single strong, divided cardinal 

 tooth, and a strong posterior lateral tooth which is grooved along its summit. 



Five specimens measure respectively 22, 28, 32, 36 and 42 mm. in length, 

 and 18, 23, 25, 26 and 29 mm. in height. 



This species is very similar in form to S. tumidus, but is less gibbous, the 

 beaks are less prominent and the posterior muscular impression is farther 

 behind the beak. S. tumidus is founded upon a single cast of the interior, and, 

 therefore a full comparison cannot be made. 



