528 Systematic Paleontology 



Nemodon cecilius n. sp. 

 Plate XX, Figs. 5-7 



Description. — Shell large for the genus, transversely elongated, inflated, 

 umbones rather high and prominent, not widely separated, the apices 

 acute and turned a little forward, slightly anterior in position ; hinge line 

 elongated, horizontal; anterior lateral margin squarely truncate, joining 

 the cardinal margin at an angle of approximately 90°, posterior lateral 

 truncation slightly oblique ; base line arcuate, broadly upcurved anteriorly, 

 more produced and sharply rounded posteriorly; shell obtusely carinate 

 from the umbones to the posterior basal angle ; external surface probably 

 sculptured with fine radial lirae separated by linear sulci which tend to 

 arrange themselves in pairs ; radial sculpture occasionally interrupted by 

 a prominent growth line, especially toward the ventral margin ; hinge 

 armature of two or three anterior and three or four posterior teeth, elon- 

 gated, transversely striate and parallel to the hinge margin, the more 

 ventral teeth shorter than those behind them and very slightly oblique; 

 between the two series a few minute cardinal denticles transverse to the 

 hinge; character of muscle scars unknown excepting for a faint linear 

 depression extending midway along the posterior area from the umbones 

 to the lateral margin implying a low buttress in front of the posterior 

 adductor ; inner basal margins coarsely crenate. 



Dimensions. — Altitude 23 mm. ; latitude 29.5 mm. ; semi-diameter 

 18.5 mm. 



This puzzling form is known only by numerous casts of the interior 

 from the type locality, and by a cast of the exterior which is probably, 

 though by no means certainly, referable to the same species. The generic 

 affinities are rather dubious; the general outline, dentition and character 

 of the sculpture (if the cast of the exterior has been correctly united with 

 those of the interior) are those of Nemodon, but it is rather large for that 

 genus. It is possible that the presence of a posterior buttress scar, a char- 

 acter which strongly suggests Cucullcea, is merely concomitant with the 

 heavier shell and is of no great systematic value. The crenulation of the 

 inner ventral margins is unknown in Cucullcea, but occurs occasionally in 



