542 SysTiarATic Paleontology 



effect, is tlie most noticeable feature of the variety. The more accurate 

 outline of the associated P. arcuata will distinguish the two forms at sight. 



Length of left valve 1.2 mm. ; height .Go mm. 



Occurrence. — ■HELDEUBEiiG Formation, Keyser Member. Cumber- 

 land. 



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



Family CYTHERELLIDAE 



Genus PACHYDOMELLA Ulrich 



Pachydomella longula n. sp. 



Plate XCVIII, Figs. 29-31 



Description. — Carapace somewhat elongate subelliptical, the ventral 

 side less curved than the dorsal, the ends nearly equal, the posterior a 

 trifle wider and very slightly truncated in the dorsal part ; valves thick and 

 very convex, the left overlapping the right all around, the overlapping 

 edge thickened and slightly prominent along the antero-dorsal and ventral 

 edges. Point of greatest thickness of left valve in the ventral half, in the 

 right valve, in the dorsal half, the resulting appearance in an end view 

 being one of oblique compression. In a ventral view, the greatest thick- 

 ness is near the middle of the posterior half, where it exceeds in amount 

 the height of the carapace. Surface of valves smooth, apparently without 

 markings of any kind. 



As near as can be determined from the character of the specimens, this 

 species conforms in every respect with the type of the genus Pachydomella 

 tumida, the only difference of consequence being that in P. longula the 

 shape of the carapace is more elongate. Jones and other authors have 

 referred somewhat similar highly inflated species to Polycope, but in these 

 forms the shell is evidently not so thick, the overlapping rim of the larger 

 valve being much less conspicuous. 



Length of complete carapace 1.15 mm.; height .60 mm.; thickness 

 .70 mm. 



Occurrence- -\l¥AStY.u-RvsM Formatiox, Keyser Member. Cumber- 

 land. 



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



