143 



of a smaller specimen, twenty-five lines ; height near the middle, thirteen 

 lines. 



In P. retroflcxa (Hisinger) the hinge-line is considerably longer than 

 the body of the shell, while the anterior wing is not acutely pointed, as in 

 this species, but truncated at nearly a right angle. 



Locality and Formation. — Arisaig, Nova Scotia. Upper Silurian. 

 Collector.— "11. C. Weston. 



Pteronitella oblonga. (N. sp.) 



Plate 9, fig. 7. 



Description. — Transversely oblong ; length, a little less than twice the 

 height. Dorsal margin straight, equal to the greatest length of the shell. 

 Ventral margin, in the middle two thirds, gently convex or nearly straight, 

 sub-parallel with the dorsal margin, curved upwards at both extremities. 

 Anterior extremity rounded in the lower third ; in the upper two-thirds 

 nearly straight or gently convex, forming with the hinge-line an angle of 

 85^ or 90°. The beaks are situated at about the anterior fifth. The 

 most projecting point of the posterior extremity is a little below the mid- 

 height ; above this the margin is slightly sinuated, and reaches the hinge- 

 line at an acute angle of about 75°. 



The left valve is moderately and broadly convex, most tumid in the 

 anterior half, somewhat flattened in the upper part of the posterior 

 portion. 



Surface with fine concentric striae, five or six in two lines, some of them 

 more strongly elevated than the others. On one specimen very fine 

 obscure radiating striae are seen, eight to ten in two lines. When partially 

 exfoliated the surface has a smoothish aspect. There are two or three 

 lateral teeth, -which occupy the middle third of the length, close below 

 the hinge-line. Several small anterior teeth. 



Locality and Formation. — Arisaig, Nova Scotia. Upper Silurian. 

 Collector.— "Y. C. Weston. 



Pteronitella curta. (N. sp.) 



Plate 9, fig. G. 



Description. — Length on the hinge-line from one -eighth to one-sixth 

 greater than the height. Dorsal margin straight, equal to the greatest 

 length or a little more. Beaks about the anterior fourth. Anterior mar- 

 gin descending nearly vertically from the hinge-line, nearly straight or 



