MARYLAND GEOLOGICAL SURVEY 119 



Cythere evax n. sp. 

 Plate XXXVI, Figs. 6-8. 



Description. — Eight valves somewhat obliquely subovate, the middle 

 halves of the dorsal and ventral edges nearly straight or very slightly 

 arcuate, the cardinal angles rounded. Left valves differ in having more 

 pronounced cardinal angles, the anterior one especially being much more 

 prominent and thicker, and in the greater length and straightness of the 

 dorsal margin. Both valves are decidedly narrower behind than in front. 

 Entire surface of valves coarsely spinulose and pitted between the spines. 

 The middle of the valves exhibits a more or less distinct vertical depres- 

 sion, separating an undefined swelling just in front of it from two more 

 ridge-like prominences lying just behind it. Of the latter the lower one 

 is the longer, and usually is prolonged anteriorly beneath the broader 

 anterior swelling. Between this ridge and the ventral edge there is a 

 furrow. 



The only described species known to us, having close relations to 

 C. evax is the C. lyelliana Bosquet, from the Eocene of Belgium. The 

 typical form of our species, however, is much shorter and differs decidedly 

 in the character, form, and marking of the swellings occupying the cen- 

 tral part of the valves. The following var. ohlongula agrees much better 

 in its outline with the Belgian species, but differs, like the typical variety, 

 in the lesser development of the anterior swelling and in the extension 

 of the spinous surface ornament over the swelling. Bosquet represents 

 the latter as perfectly smooth. 



Length of a left valve 0.78 mm., greatest height of same 0.49 mm. In 

 an average right valve of the same length the height is about 0.46. 



Occurrence. — Calvert Formation. Plum Point. Also in the Ches- 

 apeake Group at Yorktown, Va. 



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



Cythere evax var. oblongula n. var. 



Plate XXXVI, Figs. 9-10. 



Description. — This variety is distinguished from the typical form of 



C. evax by the much greater length of its valves and in the more nearly 



equal height of their two ends, this greater equality of the ends being 



