Maryland Geological Sukvey 301 



Amhocoelia umbonata Kej-es, 189], Johns Hopkins Univ. Circ, voL xi, p. 29. 

 Ambococlia um.bonata Hall and Clarke, 1893, Pal. N. Y., vol. viii, pt. ii, pL 



xxix, fig. 17; pi. xxxix, figs. 4-9. 

 Amhocoelia umbonata Schuchert, 1897, Bull. U. S. Geol. Surv., No. 87, p. 141. 

 Amhocoelia umbonata Grabau and Shimer, 1909, N. Am. Index Fossils, vol. 1, 



p. 343, figs. 438c, d. 



Description. — -Shell plano-convex, almost hemispherical contour, rather 

 wider than long ; hinge-line generally equal to the width of the shell ; 

 cardinal angles rounded. A^'entral valve very gibbous; umbo high and 

 incurved; mesial sinus strong, distinct and continuous from the beak to 

 the front; cardinal area comparatively large, arched and of considerable 

 .height. Dorsal Valve seraielliptical ; faintly convex in tlie upper central 

 part, concave below the middle and at the sides; beak scarcely elevated 

 above the hinge-line; without mesial fold, furrow or impressed line. 

 Surface marked by fine concentric striae which are sometimes crowded 

 and imbricating towards the front of the valves, and by very fine radiat- 

 ing striae which are only shown on excellently preserved specimens and 

 hence are rarely seen. 



The Maryland specimens are generally in the condition of impressions 

 altliough occasionally more or less of the shell is preserved. The im- 

 pressions of some of the ventral valves show a striated muscular area 

 toward the beak and pustulose markings over the central part of the 

 valve. Some of the specimens are fully as large as any from New York 

 and in fact broader, a condition apparently due to crushing, while others 

 are smaller than the normal ones. The species is readily identified by its 

 smooth surface, with the exception of the concentric and radiating striae 

 the latter of which are shown only on exceptionally well preserved speci- 

 mens; the very convex ventral valve with its strongly marked median 

 sinus and the nearly flat dorsal valve without ridge or furrow. 



Concerning the species found in the Onondaga fauna. Kindle says: 



The specimens which are hare referred to this species seem to show a 

 considerable range of variation which may possibly include an undescribed 

 species. Although one of the most abundant and widely distributed species in 

 the fauna it usually occurs in soft shales which furnish rather poorly preserved 

 and often flattened material. The character of this material scarcely justifies 

 detailed consideration of the types of variation observed. Hall ' states that 



"■ Pal. New York, vol. 4, 1867, p. 259. 



