616 SYSTEMATIC PALEONTOLOGY 



MODIOLUS TRIGONUS n. Sp. 



Plate XXXVI, Fig. 3 



Description. Shell thin, nacreous, equivalve, strongly inequilateral, 

 transversely elongate, suggesting a right triangle in outline, the anterior 

 margin constituting the shorter leg, the base line the longer, and the 

 posterior keel the hypothenuse; umbones prominent, acute, prosogyrate, 

 subterminal in position ; anterior margin squarely truncate in front of the 

 umbones; posterior dorsal and lateral margins gently rounded, merging 

 into one another; base line horizontal; posterior keel obtuse, persistent 

 from the umbones to the posterior ventral margin ; area behind it approxi- 

 mately half as great as that in front of it; external surface smooth and 

 lustrous, excepting for feeble incremental striations. 



Dimensions. Altitude 8.5 mm., latitude 14.5 mm., maximum diam- 

 eter 9 mm. 



The species is described from a cast of a complete individual, to one 

 side of which the shell substance still adheres. The angular outline is 

 peculiarly characteristic and nothing approaching it has been observed 

 elsewhere. 



Occurrence. MONMOUTH FORMATION. Brooks estate near Seat Pleas- 

 ant, Prince George's County. 



Collection. Maryland Geological Survey. 



MODIOLUS SEDESCLARUS n. sp. 

 Plate XXXVI, Figs. 1, 2 



Description. Shell nacreous in texture, exceedingly thin and fragile, 

 transversely elongate, .slightly wider posteriorly; umbones inflated, 

 prosogyrate, almost but not quite terminal in position; anterior end 

 obscurely truncate; dorsal margin slightly more elevated posteriorly; 

 posterior extremity obliquely rounded, the dorsal margin merging 

 smoothly into the lateral; ventral margin straight, not constricted 

 medially; umbonal ridge very prominent but evenly rounded, becoming 

 broader and lower toward the posterior ventral margin ; external surface 



