110 PALEONTOLOGY OF NEW-YORK. 



STROPHODONT^ OF THE CHEMUNG GROUP. 



Stropliudonta cayuta (n. s.). 



PLATE XIX. 

 Referred to S. interttrialu of Phiiltm, Gcol. Rep. 4th District New-York, p. 266. 1848. 



Shell semielleptical ; length and breadth about as three to five, while 

 in some specimens the length is nearly equal to the M'idth ; sometimes 

 the width on the hinge-line is twice as great as the length : cardinal 

 extremities usually more or less produced, and often auriculate. 

 Ventral valve usually of moderate convexity, sometimes but slightly 

 convex on the upper half of its length, and abruptly inflected towards 

 the front; while there is frequently a longitudinal ridge down the 

 middle. In the shaly beds, this valve is compressed and only gently 

 convex. The dorsal valve is usually very gently concave. 

 Area of the ventral valve narrow-linear, and the inner margin crenu- 

 lated for more than half its length. The dorsal area is barely a crenu- 

 lated line. 

 Surface marked by distant sharp elevated striae, between which are 

 four, five, six or more extremely fine striae, which are increased by 

 interstitial additions. 



The interior of the ventral valve, as shown in casts, is marked by a 

 broad bilobed divaricator muscular impression, which is wider than long, 

 spreading from the apex, and surrounded by an elevated rim. The occlu- 

 Bor muscular impressions are minute spots just below the apex of the 

 umbonal cavity. The divisions of the muscular impression are deeply 

 striated in the cast, and often show subordinate lobes near the margin. 

 The surface around the muscular impressions is papillose. 



This species bears much resemblance to S. inequutriata of the Hamilton group 

 in its external characters, but the ventral valve is not so regularly convex, and 

 tho dorsal valve is less concave ; while in the muscular impressions it differs con- 

 spicuously. In the coarser sandstones of the Chemung group, the specimens of this 

 sbcll are usually smaller and more convex ; while the strife are not so regular and 

 oven as in those from the more shaly portions of the formation. It is scarcely 



