LAMELUBRANCHIATA. 179 



Leptodesma longispinum. 



I'l.ATE XXI, FIUS. U, 17-1!) ; AND PLATK LXXXIX, KIGS. i-i. 



Aviriila loiii/ixpiiia. Hall. Geolofr- Surv. N. Y. : Rcji. Fnurth Dist., p. 202, fiy. S. 1848. 

 Pteriiiea lovgi^y'nia (Hall), S. A. Miller. Cat. Amor. Pal. Foss. 1877. 



Leptodesma longiKpinum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 21, tigs. 14, 17-19. 



.lau.. 1S88. 



Shell above the medium size, sub-rhomboidal ; body elongate-ovate, very 

 oblique ; length greater than the height, often nearly double ; margin from 

 the byssal .sinus to beyond the ba.se In^oadl)- rounded ; posterior margin 

 extended and al)ruptly recurved. 



Valves, in young specimens, sub-equally convex ; in older specimens the 

 right valves are somewhat less convex. 



Hinge-line straight ; length equalling or greater than the length ol' the vtilve. 



Beaks anterior, prominent, obtuse, nearly erect, arching over the hinge-line. 

 Umbonal region gibbous, subtending an acute angle. 



Anterior extension scarcely auriculate, rounded in front, straight above. 

 Wing small, very narrow-triangular ; mtirgin deeply sinuate, the bottom of 

 the sinus being close l)eneath the hinge-line; extremity protluced into tin 

 elongate spine. 



Test thin, marked by fine, irregular, concentric strijB of growth, which 

 are abruptly recurved on the post-cardinal slope, and extend along the margin 

 for nearlv half the length of the l)odv before recurving into the wino;. The 

 stria) are more elevated and crowded on the anterior side. 



The original specimen of this species has a greatest length from beak to 

 base of 42 mm., height 24 mm., hinge-line al)out 38 mm. A small right 

 valve has a length of 22 mm., height 12 mm., hinge-line 24 mm. 



This species resembles L. Rogersi, but the shell attains a larger size ; the 

 body is more oblique to the hinge-line, ;ind the posterior extremity more 

 abruptly recurved ; the wing is smaller and not extending so far down the body 

 of the valve, the sinus is narrower with the deepest portion just beneath the 

 hinge ; the spine is stronger and more distinctly defined ; the right and left 



