MARYLAND GEOLOGICAL SURVEY 117 



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

 Chesapeake Group of Virginia, on the James Eiver and at Yorktown. 

 Collections. — U. S. N'ational Museum, Maryland Geological Survey. 



Cythere exanthemata n. sp. 

 Plate XXXVI, Figs. 1-5. 



Description. — Carapace oblong subquadrate or elongate subovate, 

 obliquely rounded at the end.s, the greater curvature and prominence in 

 both cases being in the ventral half. Entire outline, excepting the 

 straight or slightly concave ventral edge, fringed with flattened spines, 

 those along the dorsal edge being of larger size than those on the ends. 

 Posterior end compressed and carrying a double series of spines, the 

 outer row sometimes occupying a low marginal ridge. Anterior end with 

 a thick border or marginal ridge within the spiny fringe, but this ridge 

 also breaks up into node-like spines in its lower third. Surface of valves 

 between these two end ridges covered with fifteen to eighteen large irreg- 

 ular blunt spines or excrescences. These spines may at first sight seem 

 to be arranged wholly without regard to any system, but on closer in- 

 spection they arrange themselves into three longitudinal rows, a rather 

 irregular one projecting over the dorsal line, a second regular series be- 

 ginning with the nodes on the lower end of the anterior marginal ridge 

 and continuing in an increasing curve across the ventral and posterior 

 slopes, and a third and much less regular row lying between the other 

 two. Several of the nodes of the middle series are grouped on the sum- 

 mit of a broad anterior swelling of the valves. Hingement strong, typical 

 for the genus. The interior marginal plate is usually wide. 



This extremely nodose and spiny carapace belongs to a section of 

 the genus of which one extreme is represented by our C. martini and 

 C. producta and the other led up to by numerous Tertiary species figured 

 by Bosquet and ending in species that Jones has included in his subgenus 

 Cythereis. We believe, however, that Cythereis should be restricted to 

 the C. ceratoptera section, of which our C. cornuta var. americana is a 

 good example, and the C. exanthemata section left with Cythere until the 

 time shall have arrived when a thorough revision of the family is possible. 

 The monographic work upon which the writers are engaged, it is hoped, 



