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PALAEONTOLOGY OP NEW-YOEK. 
STROPHODONTaE OP THE CHEMUNG GROUP. 
* Strophodonta cayuta (n. s.). 
PLATE XIX. 
Referred, to S. interstrialis of Phillips,. Geol. Rep. 4th District New-York, p. 266. 1843. 
Shell semielleptical; length and breadth about as three to five, while 
in some specimens the length is nearly equal to the width ; 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- 
sor 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. mequistriata of the Hamilton group 
in its external characters, but the ventral valve is not so regularly convex, and 
the 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 
shell are usually smaller and more convex; while the striae are not so regular and 
even as in those from the more shaly portions of the formation. It is scarcely 
