■2'A PALEONTOLOGY OF NEW YORK. 



Mytilarca (Plethomytilus) poxderosa. 



rl.ATK X.W, KIGS. -.'-7 



Mi/lilaira piDidf-roxd. 1I.U.I,. Prelim. Notice Lam. Shells, etc., p:, 2, ],. -21. I)>m'., 1800. 



IIali.. S. a. MiLiKH, Cat. Amor. Pal. F().s.s. 1S77. 

 Phthnmiililnx poinlfioxux, Hall. Pal. N. Y'., vol. v, jit. 1. Plates ami Kxiilanatioiis : PI. 30, lig«. 2-7. 



.Ian.. lS,S.i. 



Shell very large; body acutely ovate, erect ; height tliree-fiftlis the length;* 

 ventral margin nearly straight or .^lightly concave for about one-halt the 

 . length of the shell; posterior anil lia.<al margins abruptly roiuiilcil : dorsal- 

 margin gently curved to the exlreiuity of the hinge-line. 



\'alves equal, regularly convex posteriorly, gibbous above and in the 

 umbonal region. 



Hinge-line straight and short. Beaks anterior, proniincnt, rising much 

 above the cardinal line, moderately incurved. \ Ciitral side concave with a 

 slight intlation ot'tlic margin at the In'ssal opciiiiii:-. Dorsal side sub-alate. 



Test thick, marked by concentric striaB of growth uhicli liccomc lamcllo.se 

 toward the margin. 



Ligamental area finely and evenly striated longitudinally, in old shells 

 becoming very wide, indicating great thickness of the test. 



Interior unknown. 



A large specimen, which is a cast of the interior, has a leiigtli of lo") mm., 

 height 80 mm., breadth 80 nun. A small example has a length of 36 mm. 

 and height 23 mm. 



This species differs from Mijtilurca (P.) arenucca in its greater size and gil)- 

 bosity ; from M. {Pkthoynytilus) cordiformis, 11\ll=M. (P.) mijilimera, Coxrad, 

 and M (P.) ovata. Hall, from the Lower Helderberg group, in its moi'e elongate 

 form and more attenuate apex. 



Formation and localities. In the limestone of the L'pper Helderberg group, 

 Helderberg mountains, Albany county, X. Y. ; Clarence Hollow, Erie county, 

 N. v.: and in limestone (jf the same age at (Jolundius, Ohio, and Walpole, 

 Ontario. 



*Iii the mytiloid fdi'iiis the lenffth is measured from the beak to the posterior extremity and the heit'lit is 

 at rig-ht angles lo this. The breadth represents the convexity of the valves. 



