128 SYSTEMATIC PALEONTOLOGY 



ing a slight dorsal and a longer ventral overlap. Greatest thickness of 

 carapace just behind and a little below the middle. Surface closely 

 pitted. 



Distinguished b}^ its semicircular outline. 



Length of entire carapace 0.84 mm., greatest height of same 0.40 mm., 

 thickness of same 0.37 mm., length of a right valve 0.75 mm., height of 

 same 0.35 mm. 



Occurrence. — Choptank Formation. On Peach Blossom Creek, 3 

 miles southwest of Easton. Calvert Formation. Plum Point. 



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



Cytherideis longula n, sp. 



Plate XXXVII, Figs. 21-27. 



Description. — Carapace elongate, subcylindrical, slightly curved, the 

 ventral outline straight, the dorsal gently arcuate, the ends rounded, the 

 posterior one much narrower than the anterior. Eight valve the smaller, 

 more arcuate dorsally than the left and very gently sinuate ventrally. 

 Left valve with a sharply defined though narrow marginal rim on the 

 anterior end; right valve with a similar flattened border on both ends. 

 Surface pitted or reticulated, some variation in the size and distribution 

 of the pits being shown among the abundant specimens before us. Hinge- 

 ment as demanded by the genus. 



Distinguished from C. subcequalis by its greater length, unequal ends 

 and marginal rims. C. semicircularis is shorter, a little thicker and dif- 

 ferently outlined in lateral and ventral views. C. cylindrica has wider 

 and more equal ends, a finer surface reticulation, and a less arcuate dor- 

 sal edge. The general aspect of C. longula is also very similar to that of 

 the Eocene Bytkocypris parilis Ulrich, but there are recognizable differ- 

 ences in their respective outlines, the dorsum of the Bythocypris being 

 more arcuate and its ends more equal and without the flattened borders 

 which are always present on the valves of C. longula. The reticulation 

 or pitting of the surface also is a more conspicuous feature in the latter. 



Length of entire carapace 0.90 mm., greatest height of same 0.36 mm., 

 thickness of same 0.33 mm., length of a left valve 0.95 mm., greatest 

 height of same 0.37 mm., length of a right valve 0.90 mm., greatest 

 height of same 0.34 mm. 



