LAMELLIBRANCHIATA. 515 



MEGAMBONIA, Hall. 1859. 

 Megambonia cardiiformis. 



PLATE I.II, FIGS. 1-8. 



Pterinea cardiiformis. Hall. Geol. Rep. Fourth Dist. N. Y., p. 172, fig. 1. 1843. 

 Megamhiiiiia cardiifomiis. Hall. Twelfth Ann. Rep. N. Y. State Cab. Nat. Hist., p. 13. 1859. 



Pal. N. Y., vol. iii, p. 272. 1859. 



" subcardiformis, " S. A. Miller.* Cat. Am. Pal. Foss., p. 194. 1877. 



" " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations :* PI. 52, figs. 1-8. 1883 



Shell large, cardiiform, higher than long ; basal margin regularly rounded ; 

 posterior margin curving abruptly upward to the cardinal line ; cardinal 

 margin sub-alate posteriorly. Anterior end short, auriculate, separated from 

 the body of the shell by a distinct byssal sinus, which is conspicuously 

 marked in the margin of the shell. 



Valves regularly convex, very gibbous in the umbonal region ; the depth 

 of the two valves is somewhat less than the length of the shell. 



Beaks sub-anterior, incurved ; umbo prominent, rising above the hinge-line. 



Test thick, marked by concentric lamellse of growth and by fine radiating 

 striae. 



Anterior muscular impression large and deep, situated within the auricle 

 and occupying a large part of the lobe. Posterior muscular impression large, 

 situated near the post-cardinal extremity. There is an obscure indication 

 of a lateral tooth on the post-cardinal angle. 



A large and nearly perfect specimen has a length of 59 mm. and a height 

 of 73 mm., with a depth to both valves of 46 mm. A larger specimen has 

 a height of 76 mm. and a transverse diameter of 64 mm. 



This species somewhat resembles M. belUstriata of the Oriskany sandstone, 

 but differs in its greater gibbosity, shorter and more oblique sinus, separating 

 the auricle. 



Formation and locality. In the upper member of the Corniferous limestone 

 at Clarence Hollow, Erie county, N. Y. 



• The reference of this species under the specific name of subcardi/ormis has come from copying an 

 erroneous manuscript record of the species. 



