MARYLAND GEOLOGICAL SURVEY 123 



Occurrence. — Calvert Formation. Plum Point. Chesapeake Group. 

 James Eiver and Yorktown, Va. 



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



Cythereis alaris n. sp. 

 Plate XXXVIII, Figs. 34-36. 



Description. — Carapace rather elongate, somewhat acuminate-subovate, 

 the anterior half about one-third wider than the posterior; dorsal and 

 ventral margins nearly straight, anterior outline broadly rounded, the 

 dorsal angle indefinite; posterior edge most prominent near the middle, 

 the lower half rounding regularly up from the ventral margin, the upper 

 half distinctly concave and sloping strongly forward to the obtusely 

 angular posterocardinal angle. Excepting the dorsal halves of both the 

 anterior and posterior edges the rest of the outline is fringed with flat- 

 tened spines, of which those along the dorsal edge and one on each side 

 of the middle of the ventral edge are much larger than the others. Of 

 the five or six spines along the dorsal edge the one just in front of the 

 postcardinal angle is the largest and most prominent, especially in a 

 view of the back. Valves on the whole appearing compressed, but ex- 

 hibiting a broad swelling that takes up most of the anterior half of the 

 surface, while behind it and beneath its center there is a wing-like ridge 

 the crest of which is broken up into four or five unequal spines directed 

 obliquely downward and forward. Posterior fourth of surface depressed, 

 with a small tubercle near the postdorsal angle. Surface smooth. 



Length of a left valve 0.34 mm., greatest height of same 0.50 mm., 

 thickness at anterior swelling 0.18 mm., to summit of ventral ridge 

 0.22 mm. 



The low swelling of the anterior half of the valves brings this well- 

 marked species into comparison with Bosquet's Cythere dumontiana, with 

 which it agrees rather closely also in outline and in the length of the 

 spiny ventrolateral alation. That species, however, differs in wanting 

 the marginal spines on the ventral and anterior edges, and in this respect 

 our species agrees very closely with C. fimbriata Miinster, which Lienen- 

 klaus says is the same as Cythere ceratoptera Bosquet. C. alaris there- 

 fore appears to occupy an intermediate position between the two Euro- 

 pean species mentioned. 



