MARYLAND GEOLOGICAL SURVEY 113 



middle of the ventral edge. Left valve generally a little higher than 

 the right, which it overlaps ventrally, and enclosed, except along the 

 dorsal edge, by a thick rim, heaviest anteriorly and barely distinguish- 

 able in the anteroventral region. Usually there are no marginal spines 

 at either end of this valve. Both valves exhibit a broad swelling, occu- 

 pying the greater part of the anterior half, but it is nearly always more 

 conspicuous on the left valves. The surface of the right valves, on the 

 contrary, seems to be more protuberant near the posterior margin than 

 the left. Occasionally the right valve bears also a small central pro- 

 tuberance. Surface of both valves reticulate or simply pitted, the pat- 

 tern, as shown by the illustrations, being somewhat variable. 



Length 0.75 mm. to 0.8 mm., height 0.39 mm. to 0.42 mm. 



JSTamed for Dr. G. C. Martin, of the Maryland Geological Survey. 



Occurrence. — Choptank Formation. Pawpaw Point, Peach Blossom 

 Creek. Calvert Formation. Plum Point. Also at Yorktown, Va., in 

 the Chesapeake Group. 



Collections. — U. S. ISTational Museum, Maryland Geological Survey. 



Cythere dorsicornis n. sp. 

 Plate XXXVI, Fig. 16. 



Description. — Of this species we have two varieties, both apparently 

 rare. The Calvert form, with its single conical node or spine near the 

 middle of the posterior half of the dorsum, should be considered as the 

 typical variety. The general aspect of the valves of these two varieties 

 is greatly like that of the associated C. martini. Still they may be dis- 

 tinguished even without taking into consideration the conical nodes 

 which are wanting in that species. Thus, in C. dorsicornis the swelling 

 of the anterior half of the valves is a little larger and moreover is divided 

 into two parts by a curved sulcus. Next the posterior extremity is more 

 produced and compressed while the length of the valve is proportionately 

 less and the height of the anterior end slightly greater. Finally, the 

 arrangement of the surface pits varies from- that generally obtaining 

 in C. martini. These differences are all of subordinate value and we 

 were inclined at first to rank the Calvert form as a variety under that 

 much more abundant species. When, however, the second variety was 



