75 



Hating at the semicircular ridge witli a strongly elevated projection. At 

 the hinge-line there is a sort of foramen passing between the dental soc- 

 kets and forming a concave groove across the area nearly to the beak. 

 On each side of this foramen there is a small projection similar to those 

 that are seen in the dorsal valve of OrtUs. It is barely possible that 

 these these may represent the divaricator processes, but at present I am 

 inclined to the opinion that they do not. I think the muscles were attached 

 to the surface at the bottom of the groove which passes between them. 



Surface with fine striae alternating ia size exactly as in StropJiomena 

 alternata. 



Width from 4 to 6 lines. Length from 3 to 4 Hnes. Area of ventral 

 valve from ^ to | of a line in height at the foramen. 



Locality and formation. — Point L^vis. In the upper part of lime 

 stone No. 2, Quebec group. 



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



Orthis gemmicula. (N. sp.) 



Fig. 69. Fig. To. Fig. 71. 



Fig. 68. — Orthis gemmicula; a, b, c, three views of a specimen enlarged. The 



small figures show the natural size. 



69. — 0. Tritonia ; a, dorsal valve ; 5, interior of the same. 



VO. — O. orthambonites ; a, b, c, ventral, dorsal and side views. 



Yl. — 0. Euryone; a, b, ventral valve. 



Description. — Shell very small, transversely oblong or semioval ; hinge 

 line equal to or a little greater than the width of the shell ; cardinal 

 angles either rectangular or slightly produced, forming small acute ears ; 

 sides either straight or gently convex, and subparallel or converging to- 

 wards the front; front margin either rounded, straight or sinuate, and 

 often with a deep notch in the middle. Ventral valve very convex, most 

 elevated in the upper half and along the middle, where it is often obtusely 

 carinated, descending with a gently concave or somewhat flat slope to the 

 sides ; cardinal angles compressed ; beak small, pointed, depressed to 

 about half the height of the valve ; area about half the height of the valve, 

 forming with the plane of the lateral margin an angle of about 110° ; 

 foramen scarcely so wide as high; dorsal valve convex with a deep 

 angular sinus which divides the valve into two tumid lobes. Surface when 



