Maryland Geological Survey 447 



duced into a lingiilaie extension at the anterior margin. The general 

 flatness of this valve is moi'e marlced in young shells, the anterior deflec- 

 tion becoming prominent with the increase of age. The brachial valve 

 has a full beak curved into the delthyrium of the opposite valve, and is 

 elevated medially into a broad ridge-like concavity terminating in a fold on 

 the anterior margin. The lateral slopes are gently concave. The surface 

 of both valves is smooth or bears only concentric growth lines. 



" On the interior the apophyses and impressions are those characterizing 

 the genus. Particularly well developed is the broad, flabellate, muscular 

 scar of the pedicle valve and the median septum of the brachial valve, 

 extending for more than half the length of the shell. The brachial valve 

 resembles in its less extreme conditions that of the associated specimens 

 of M. lata, but the shell taken in its entirety cannot be confounded with 

 any other species." Clarke, 1900. 



M. lentiformis is generally raucli smaller than M. lata. It is also far 

 more transverse, the ventral sinus more pronounced with the tongue- 

 shaped extension more protruding into the dorsal fold, and the dorsal 

 umbo more suddenly heaped than in M. lata. In the Lower Oriskany the 

 specimens are all small, but in the Upper Oriskany they attain a width of 

 30 mm. and a length of 25 mm. 



Occurrence. — Oriskany Formation, Shriver Member. North 

 Branch, common, Winchester Eoad near Cumberland, common. Ridgely 

 Member. Cumberland. 



Collections. — Maryland Geological Survey, U. S. JSTational ^luseum. 



Meristella rostellata n. sp. 

 Plate LXXIV, Figs. 14, 15 

 Description. — Shell attaining a large size, elongate, rostrate, and sinu- 

 ate. Outline variable from elongate oval to subquadrate, with the greatest 

 width in the anterior third. Anterior margin a little drawn out into a 

 Ungulate extension owing to the ventral sinus. Ventral valve depressed 

 convex except in the rostral region, which is very convex, and the large 

 beak incurved over the dorsal shell; sinus narrow, shallow, and defined 

 only in the anterior third, terminating in a short but well-defined Ungulate 



