II i;i.ni:i;i!i:i!(i Fok.matiox in ^Maryland 15 



front margin. On the younger shells the ribs are usually finer and more 

 angular and the lateral ones often have a slight anterior curvature as they 

 approach the margin. As the shells increase in size this peculiar curva- 

 ture of the ribs becomes more and more coiispicuous, and can always be 

 detected to a greater or less degree in the adult individuals, and it is always 

 more conspicuous in the brachial than in the pedicle valve. Just anterior 

 to the beak in both valves there is a small area where the radiating ribs 

 are obsolete. In addition to the radiating ribs, both valves are marked by 

 exceedingly fine, concentric lines. 



"' The dimensions of a large individual are : Length 14 mm. and width 

 22 mm.'' Weller, 1903. 



This species is an exceedingly variable one in form. The larger speci- 

 mens may be recognized by the peculiar curvature of the radiating ribs, 

 while in the smaller or younger specimens the ribs may be straight or in 

 others have the peculiar curvature. 



Occurrence. — Helderbeeg Fokmation, Keyser Member. Dawson, 

 Cookerly, Cash Valley, Devil's Backbone, Breakneck Hill, Flintstone, 1| 

 miles east of Eush, Hancock, Tonoloway, Maryland; Keyser, West 

 Virginia. 



Collection. — Maryland Geological Survey. 



Choxetes jerseyensis yar. spixosus n. var. 

 Plate LXI, Fig. 20 



Description. — This variety of Clionetes jersey ensis differs in being 

 larger and having a greater number of marginal spines on each side of 

 the beak. Pedicle valve slightly more convex. The radiating ribs are 

 coarser and somewhat nodose. 



Length 17 mm. ; width 22 mm. 



Occurrence. — Helderbeeg For:matiox', Keyser Member. Hancock. 



Collection. — Maryland Geological Survey. 



