Marylaxd Geological Survey G0;3 



This little sliell is very widely (lilTuscil tliruugii the Maryland Cheniiuig 

 strata; frequently heaped together in masses which constitute thin seams 

 through the sandy matrix. So far as can be ascertained the species shows 

 no deviation from its usual expression and there is no evidence to indi- 

 cate the propriety of applying to this shell the varietal name gregaria 

 used by Hall for the Clieaiung fonii. The eurvcd. arched ventral valves 

 show a sharply defined median groove and the rotund lateral slopes bear 

 only traces of concentric stria?. The dorsal valves are gently convex 

 about the umbo, and flattened or depressed toward the margins. The 

 interior characters are often well preserved on the casts showing the 

 peculiar muscular scars of tlie dorsal valve and the deep pedicle impres- 

 sion of the ventral valve. It has been difficult, on account of this mode 

 of preservation, to obtain well-defined impressions of the exterior of the 

 shells. 



This species is very abundant in the Chemung but rarely occurs below 

 that horizon. The iirst incoming of the Chemung fauna is usually indi- 

 cated by a profuse development of this form in beds which lie a short 

 distance below the lowest beds containing Spirifer dAsjunctus, while it 

 continues to be abundant in the higher horizons. The lowest beds con- 

 taining Ainhoca'Vui urnhonata are here considered to be a part of the 

 Chemung. 



Occurrence. — Jennings Formation, Chemung Member. Oakland- 

 Altamont Road; Deer Park; Mountain Lake Park; Oakland-Eedhouse 

 Eoad; Trout Eun section; Allegany Grove 1153 ?, 12.50 abundant, 1310 

 abundant, 2076 abundant; Ellerslie, Pennsylvania, 1474 aljundant; 2% 

 miles south of Eound, upper Tropidoleptiis zone, abundant; Williams 

 Eoad, on Polish Mountain, 1837; National Eoad, on Polish Mountain, 

 1957, 2503; near Pennsylvania-Maryland state line west of Green Eidge 

 in upper TropiSoIeptus zone ? abundant; Town Creek, 3023 abundant, 

 2122, 2691 abundant, 2750, 3384, 3453 ; Fifteenmile Creek, 1 mile above 

 Little Orleans, 2215; National Road, on Green Eidge, 3400 abundant; 

 Woodmont, 1667; Hancock, 2223 abundant; Millstone, 2225, 2322 com- 

 mon. WooDMONT Member, Naples ? Fauna. Allegany Grove, 475. 



Collection. — Maryland Geological Survey. 



