258 PALEONTOLOGY OF NEW YORK. 



Mytilarca Chemungensis. 



PLATE XXXTT. FIGS. 8-11, 13, 14 



Inoceramiis Chemangensis, Coxrad. Jour. Acail. Nat. Sci., Phila., vol. 8, p. 24G, pi. 13, fig. 9. 1842. 

 Mytilus Chemungen.H.1 (Conrad), Phillips and Salter. Memoirs Geolog. Surv. of Great Britain, vol. 2, pt. 



1, p. 365, pi. 20. figs. 10, 11. 1848. 

 Mytilarca Chemungensv! (Conrad), Hall. Prelim. Notice Lara. Shells, etc., p. 23. 1869. 



<■ " " '• Pal. N. Y., vol. V, pt. 1. Plates and Explanations : PI. 32, figs. 



8-14 (pi. 33, fig. 8, in eiTOr). Jan., 1883. 



Shell of medium size, much elongated from beak to posterior extremity ; body 

 narrow, elongate-ovate, sub-arcuate ; length averaging twice the height but 

 with considerable variation ; ventral margin nearly straight, slightly concave 

 for nearly three-fourths of the length ; basal margin abruptly rounded ; 

 dorsal margin gently curving. 



Valves equal, convex in the posterior part, gibbous above ; the greatest 

 convexity a little above the middle. The ventral unibonal slope is obtuse, 

 sloping nearly vertically to the ventfal margin. 



Hinge-line short, oblique. Beaks elevated, acute, anterior, slightly 

 incurved. 



Test thin, marked by very fine concentric striae, which toward the mar- 

 gins are often fasciculate and sub-imbricating. 



Ligamental area finely striated longitudinally. Cardinal teeth small, 

 diverging, situated immediately under the beak. Other characters of the 

 interior unknown. 



The proportions of length and height are subject to great variation. A 

 short example has a length of 26 mm. and height 17 mm. A specimen of 

 ordinary proportions has a length of 5.3 mm. and height 26 mm. An 

 elongate form has a length of 56 mm. and height 22 mm. 



This species differs from M. carinata in its comparatively longer form ; the 

 ventral umbonal slope less angular ; and the body less arcuate. 



Formation and localities. In tlic middle portion of the Chemung group, 

 Rockville, Hobbieville and Phillipsburgh, Alleghany county, and East 

 Randolph, Cattaraugus county, N. Y. 



