MARYLAND GEOLOGICAL SURVEY 367 



There seem to be no good generic grounds for separating M. virginicus, 

 M. dalli and M. ionensis from each other, and hence, largely from 

 the pronounced Gregariella features of M. ionensis, it has seemed best to 

 place the three species under that section of Modiolus rather than to con- 

 sider them under Modiolaria. 



Length, 7.5 mm. ; width, 4 mm. ; diameter, 2.3 mm. 



Occurrence. — St. Marys Formation. St. Mary^s River. 



Collection. — Maryland Geological Survey. 



Modiolus dalli n. sp. 

 Plate XCVII, Figs. 9, 10. ' 



Description. — Shell small, thin, delicate, somewhat perlaceous, vaulted, 

 elongated; posterior dorsal margin subangulated at end, rest of margin 

 rounded, posterior margin and posterior portion of basal margin strongly 

 erenulated, anterior and dorsal margins partly faintly crenulated, mid- 

 basal margin smooth; ligament groove narrow, straight; interior of shell 

 smooth; posterior and anterior areas of outer surface of shell sculptured 

 with fine rounded radial threads reticulated or granulated by concentric 

 lines, near posterior basal margin additional fine radial lines pro- 

 duced by branching or by intercalation between longer lines; posterior 

 slope rudely undulose, especially in its superior portion ; slope from beak 

 to posterior basal margin distinctly elevated and ridged, anterior to which 

 the surface is depressed or almost grooved at junction of sculptured and 

 smooth areas; smooth area crossed by irregular concentric growth striae; 

 anterior radial sculpturing faint. 



This species is much more produced, is thinner and more finely and 

 delicately sculptured than M. virginicus, which is a compact, stout and 

 somewhat coarsely sculptured species. It is named in honor of Dr. W 

 H. Dall. 



Length, 8.9 miu. ; height, 4.5 mm. ; diameter, 2 mm. 



Occurrence. — Choptank Formation. Pawpaw Point. 



Collection. — Maryland Geological Survey. 



