76 



very perfect with fine concentric striae crossed by stronger radiating 

 striae which give to the shell a beautifully cancellated appearance ; when 

 not perfect, either smooth or with only the radiating striae. 



Width from 1 to 1|- lines ; length from | to J less than the width. 



This is the smallest species of Orthis known in the Lower Silurian 

 rocks of Canada. Numerous perfect single valves and a few with both 

 valves connected were procured by treating fragments of limestone hold- 

 ing the silicified fossUs with acid. The form varies somewhat after the 

 manner of 0. Lynx, being sometimes transversely oblong with the sides 

 subparallel and often narrowed towards the front, and with or without 

 short ears at the cardinal angles. The front is often rounded, but in gen- 

 eral it is either somewhat straight or with a wide angular notch in the 

 middle crossed by the deep mesial sinus of the dorsal valve. In the fora- 

 men there is seen in several of the specimens a projection which seems to 

 be a large divaricator process, but in others this is not seen. 



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

 stone No. 2, Quebec group. 



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



Orthis Tkitonia. (N. sp.) 



Fig. 69. a, b. 



Lescription. — Dorsal valve transversely sub-elhptical, hinge-line nearly 

 equal to the greatest width of the shell, cardinal extremities rounded but 

 somewhat angular, sides gently convex, anterior angles broadly rounded, 

 front margin very gently convex. The shell is moderately convex, most 

 elevated at about one third the length from the beak, compressed near the 

 cardinal angles ; beak small, incurved down nearly to the plane of the 

 margin ; umbo broadly rounded, projecting a httle over the hinge-hne. 

 Surface with angular bifurcating ribs, several times divided between the 

 umbo and the front margm, becoming very fine on approaching the cardi- 

 nal angles, from four to six in the width of two lines at the front maroin. 

 There is a very obscure mesial depression, which scarcely affects the con- 

 tour of the surface but is still perceptible, running all the way to the beak, 

 on approaching which it becomes attenuated to a smgle impressed line. 

 The area is small, consisting simply of the thickness of the shell ; the fora- 

 men wide but very low. 



In the interior of the valve the anterior pair of occlusor muscular im- 

 pressions consist of two small but deep pits of an oval shape, one hne in 

 length and half a hne in width, situated close to the median Ime and about 

 their own length from the beak. Above each of those are two very small 



