716 SYSTEMATIC PALEONTOLOGY 



valve ; resilial pit in the right valve broad but rather shallow, the solitary 

 tooth subumbonal in position, stout, obtusely conical ; receiving socket in 

 left valve also subumbonal, long but not very deep; adductor muscle scars 

 relatively long, rather indistinct; pallia! sinus broad, shallow; pallial line 

 rather near the basal margin. 



Dimensions. Altitude 6.9 mm., latitude 11.4 mm., maximum diameter 

 of double valves 5 mm. 



This is the largest of the Corbulce in the Maryland Cretaceous. 



Occurrence. MONMOUTH FORMATION". Brooks estate near Seat Pleas- 

 ant, Prince George's County. 



Collection. Maryland Geological Survey. 



COHBULA TERR AM ARIA 11. sp. 



Plate XLII1, Figs. 8-10 



Description. Shell rather small, moderately inflated, ovate-trigonal in 

 outline, inequilateral, inequivalved ; medial and posterior ventral margins 

 and the posterior lateral margins overlapping; anterior ends broadly 

 rounded in both valves; posterior keel more produced in the right valve, 

 the area behind it wider than in the left and obtusely angulated near the 

 dorsal margin; base line in the right valve quite strongly arcuate ante- 

 riorly and medially, recurved and slightly contracted toward the posterior 

 keel ; in the left valve feebly and somewhat obliquely arcuate ; umbones 

 not very prominent, of equal altitude in both valves, flattened upon their 

 summits, incurved and prosogyrate; posterior area cut off by a sharply 

 rounded ridge which extends from the apices to the posterior basal line ; 

 external surface of both valves sculptured with about thirty rounded 

 obtuse concentric ridges, which are strongest upon the disk but which per- 

 sist with diminished strength across the posterior area, growing gradually 

 finer and closer toward the umbones; ligament internal, supported by a 

 chondrophore in the left valve, which judging by the resilial pit in the 

 right is rather small ; cardinal tooth in right valve stout and conical, sub- 

 umbonal in position; adductor impressions distinct, the anterior a little 

 ventral and the posterior a little dorsal to the median horizontal; pallial 



