Makylaxd Gjcoi.ogical SuiiVKY 587 



Chemung fauna by shells which attain considerable diniensionp, with 

 depressed or slightly convex ventral valves and highly convex, rotund 

 ddrsiil valves and with the surface rather coarse ril)l)ed and bearing strong 

 distant concentric varices extended into free lamell:>3 though these 

 lamellce are seldom shown. The expression of the species is the same as 

 that under wdiich it appears in the Ithaca and Chemung rocks of New 

 York. 



On the interior the ventral valve bears large flabellate diductor scars 

 surrounding an elongate cordiforrn median scar. This muscular surface 

 is surrounded Ijy a pustulose pallial area. The teeth are elongate, diver- 

 gent and crenulate on their outer surface and on the dorsal valve the 

 dental grooves of the hinge plate are similarly crenulated. The muscle 

 scars in the dorsal valve are quadripaitite and usually obscure. 



Length 25 mm.; width 32 mm. 



This species is locally abundant in tlie Ithaca, less frequent in the 

 Parkhead and Chemung. 



Occurrence.— J-K^-Ni^GS Formation, Chemung Member. Xational 

 IJoad west of Frostburg; Oakland-Altamont Eoad ; Trout Run section; 

 Oakland-Eedhouse Road; near EUerslie, Pennsylvania; Williams Eoad, 

 on Polish Mountain, 3042 ; Town Creek, 3384, 3453. Parkhead Mem- 

 ber. Williams Road, on Polish Mountain, IGGO: 2 miles west of Paw- 

 paw, 1794; 21 4 miles above mouth of Sideling Hill Creek. Woodmont 

 Me:mber, Ithaca Fauna. Woodmont 500 ; Hancock 690 common ; 

 Hancock-Harrisonville Road, 726, 880 abundant, 944 abundant, 988; 

 Berkeley Springs, West Virginia, 712 abundant, 755, 786; Yellow 

 Springs, West Virginia, 



Collection. — Maryland Geological Survey. 



Atrypa spinosa Hall 

 Plate LV, Figs. 12, 13 



Atrypa spinosa Hall, 1843, Geol. of N. Y. Rept. 4th Dist., p. 200, figs. 1, 2. 

 Atrypa spinosa vel. aspira Hall, 18G7, Pal. of N. Y., vol. iv, p. 322, pi. liiia, 



figs. 1-14, 18, 24, 25. 

 Atrypa spinosa Grabau and Sliimer, 1909, N. Amer. Index Foss., vol. i, p. 311, 



figs. 391, 392. 



