68 PALAEONTOLOGY OF NEW-YORK. 



t 



. Var. A. Streptorliyiiclius pandora. 



PLATES IV & IX. 



S(r(p(orAyiirAtu pandora : Billinos, Canadian Journal, p. 226. 1860. 



>• « BitLiNQS, Geology of Canada, p. 369. 1863. 



Comp&re Streptorhynehut [Sirophomena] woolworthana , Hall, Tenth Report State Cabinet, p. 8. 1857. 



•• «< <■ <• Hall, Pal.Ncw York, Vol. iii, pa. 192, pi. 16, 



f. 1 & 2. 1859. 



This shell, in its symmetrical form is somewhat semioval, the length and 

 width being about as four to five. A well-formed specimen, of one inch 

 in length, measures one and a quarter inches in width : hinge-line 

 equal to the greatest width of the shell, and forming right angles 

 with the sides, which are usually nearly straight ; the front broadly 

 rounded : cardinal extremities sometimes a little salient, and some- 

 times rounded. Ventral v.alve convex on the umbo, the greatest con- 

 vexity being at the apex, from which the surface slopes in a regular 

 plane to the sides and middle of the shell, becoming slightly concave 

 below the middle and along the front : area large triangular, extending 

 the entire length of the hinge-line, and inclining over the dorsal valve 

 at an angle of about 110°. The foramen has been large, about twice as 

 wide at the base as the height, and is closed by a convex deltidium. 

 Dorsal valve moderately convex, a little flattened towards the cardinal 

 extremities, and curving towards the sides and front ; area narrow and 

 linear. 



Surface striated by numerous fine strongly elevated striae, which are 

 increased by bifurcation and interstitial additions, and crenulated by 

 fine closely arranged concentric striae. 

 The interior of the ventral valve is marked by strong dental lamellae 



and a broad flabellate muscular impression, which, in well-preserved 



flpecimens, shows the cordiform imprint of the adductors, and a short 



low mesial septum in the upper part. 



The interior of the dorsal valve preserves deep dental sockets, with 



thick socket-plates which support the duplicate cardinal process, each 



division of which is grooved at the extremity. There is likewise a small 



