134 SYSTEMATIC PALEONTOLOGY 



Occurrence. — Chesapeake Group. James Eiver, Va. Probably also 

 in Maryland. 



Collection. — U. S. National Museum. 



Genus CYTHERIDEA Bosquet. 



Cytheridea subovata n. sp. 

 Plate XXXVII, Figs. 1-8. 



Description. — Carapace apparently somewhat variable in outline, sub- 

 ovate, a little narrower posteriorly than anteriorly : the ventral outline 

 less arcuate than the dorsal. Eight valve narrower than the left, both 

 its ventral and dorsal edges being overlapped; lower half of anterior 

 margin usually carrying six to eight small spines, diminishing in size 

 downwardly, while only three or four similar spines occur on the post- 

 ventral corner. No spines have been observed on the left valve. Sur- 

 face of valves usually nearly evenly convex and appearing quite smooth, 

 but on close inspection a few very small scattered punctas may be ob- 

 served. 



The specimens before us indicate two varieties that, should they prove 

 constant enough, may later on be distinguished. Of the one we have 

 seen only the right valve shown in fig, 1 on plate XXXVII. This differs 

 from the typical form in the more uniform curve and relative bluntness 

 of the posterior outline, and in the obtusely conical instead of almost 

 uniformly convex elevation of the surface. 



The second variety seems to be normal in all respects save that the 

 left valves, which alone we have observed, are considerably narrower. 



The crenulated hinge, as well as all other characters of the valves of 

 this species, very clearly indicate Cytheridea, but it does not appear to 

 have very close aflBnities with any of the described species of the genus. 

 In a broad way these fall into two groups, the first, including such forms 

 as G. miilleri, C. dehilis and C. sorhyana, being characterized by an acu- 

 minate posterior extremity, while the second, embracing such species 

 as C. perforata, C. pinguis and C. incrassata, the posterior end is blunt 

 and the anterior end so wide that the outline is subtriangular, C. subovata 

 evidently occupies an intermediate position though its affinities probably 

 are with the second group rather than the first. 



