7T 



pits which may represent the posterior pair. There is no divaricator 

 process, the umbo being simply hollowed out into a triangular cavity, to 

 the bottom of which the muscles for opening the valve were attached. - 



Length 5 lines ; width 7 lines. 



Ventral valve unknown. 



Of this species we have only a single but very perfect dorsal valve. It 

 resembles the dorsal valve of 0. plicatella, but is not so convex. It dif- 

 fers from 0. pectenella and 0. subquadrata in external aspect, while in 

 the interior it exhibits no divaricator process. I know of no species in 

 the Lower Silurian rocks which has the muscular impressions situated so 

 near the hinge-line. 



Locality and Formation. Point LeVis. In the upper part of the lime- 

 stone No. 2, Quebec group. 



Collectors. Sir W. E. Logan, J. Richardson. 



ORTHIS ORTHAMBONITES. (Pander.) 



ORTHIS CALLIGRAMMA, var. ORTHAMBONITES. MURCHISON and DE VERNEUIL. Geol. Rus. 

 Vol. II. p. 207, PL XIII. fig. 8. 



Fig. TO. o. &, c. 



Description. Shell rather small, coarsely ribbed, semioval or suborbi- 

 cular ; ventral valve convex, and dorsal valve nearly flat ; hinge-line 

 straight and nearly equal to the greatest width of the shell ; front angles 

 and margin broadly and nearly uniformly rounded ; sides in the upper 

 half of the shell somewhat straight or gently convex. Ventral valve 

 strongly convex, most elevated in the upper half; beak incurved down 

 nearly to the level of the plane of the lateral margin ; umbo prominent 

 neatly rounded, overhanging the hinge-line ; cardinal angles somewhat 

 compressed ; area small, concave, not extending the whole width of the 

 shell ; foramen small, about as wide as it is hig^. Dorsal valve nearly flat, 

 gently convex in the upper half, sloping to the cardinal angles and front 

 margin, with a wide barely perceptible mesial sinus ; area small, nearly 

 at a right angle to the plane of the margin ; foramen small, extending to 

 the beak ; the latter scarcely distinct from the hinge-line. In the inte- 

 rior of this valve the margin all round the front and lower half of the sides 

 is deeply crenulated by the coarse ribs. In the upper half there is a low 

 obtuse mesial ridge which on approaching the foramen divides, a branch 

 going to each of the crural processes. In the foramen there is a thin 

 plate-like divaricator process. The occlusor muscular impressions are 

 small and situated close to the mesial ridge in the upper half of the valve. 



