LAMELLIBRANCHIA TA. L79 



Leptodesma longispinum. 



PLATE XXI. FIGS. 14. 17-19; AND PLATE I.XXX1X, FIGS. 2-4. 



Avieula longltpina, Hall. Geolog. Sutv. N. Y. : Rep. Fourth Dist., p. 262, fig. 3. 1843. 



Pterinea longispina (Hall), S. A. Miller. Cat. Amei\ Pal. Foss. ]s77. 



Leptodesma longispinum, Hall. Pal. N. V.. vol. v. pt. 1. Plates ami Explanations: PI. 21. figs. 14, 17-19. 



.Ian.. ISS:S. 



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 base broadly rounded; posterior margin 

 extended and abruptly 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 of the valve. 



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 ; margin deeply sinuate, the bottom of 

 the sinus being close beneath the hinge-line; extremity produced into an 

 elongate spine. 



Test thin, marked by fine, irregular, concentric stria? of growth, which 

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

 for nearly half the length of the body before recurving into the wing. The 

 striae 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 about 38 mm. A small right 

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



This species resembles L. Roger si, hut the shell attains a larger size ; the 

 body is more oblique to the hinge-line, and 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 



