28 Trans. Acad. Sci. of St. Louis. 



species cooperensis. The generic reference, however, is more 

 of a personal opinion. Personally the writer leans toward 

 the opinion that all these shells are really a single species, but 

 that the essential generic characters have been observed in 

 but comparatively few individuals, therefore the Northview 

 specimens are referred to Pernopeclen cooperensis. 



MODIOMORFHA NORTHVIEWEN8I8 n. sp. 

 PI. IV. f. 19. 



Shell small, subovate in outline, length of the type speci- 

 men 12^ mm. and the height 10 mm. Basal margin straight 

 in the central portion, curved upward at the anterior and 

 posterior extremities ; anterior margin short, curved below 

 into the basal margin and nearly straight above; posterior 

 margin broadly rounded, curved from the basal to the 

 middle of the dorsal margin. Hinge-line arcuate, oblique, 

 extending backward to about the middle of the dorsal mar- 

 gin. Valves moderately convex, becoming somewhat gibbous 

 in the umbonal region. Beaks small, pointed, placed at the 

 extreme anterior end of the shell. Surface marked by fine 

 subobsolete concentric striae. 



Remarks. This species resembles, in a measure, the young 

 individuals of the New York Hamilton species 31odiomorpha 

 concentrica^ but it has the beaks placed more anteriorly and 

 lacks the strong concentric markings of that species. The 

 extreme anterior position of the beak seems to distinguish it 

 from any of the remaining species of the genus. 



Ptychodesma. — Cf . P. MINOR Hall. 



PI. IV. f. 18. 



Shell small, subovate in outline, the largest specimen 

 observed having a length of 17 mm., the height about two- 

 thirds of the length. Basal margin regularly rounded ; poste- 

 rior margin broadly rounded from the basal into the dorsal 

 margin; anterior end short, regularly rounded below the 

 beaks. Hinge-line short and oblique. Valves regularly 

 convex, becoming somewhat gibbous in the umbonal region. 

 Beaks anterior, prominent, and incurved. Surface marked 

 by fine concentric striae. Anterior muscular impression of 

 moderate size, situated close to the anterior margin under the 



