302 THE CANADIAN NATURALIST. [Vol. vH. 



ibrmly convex, gradually sloping upwards nearly (if not quite) 

 to the hinge-line. The dorsal margin is not perfect in the speci- 

 men figured, but judging from the direction of the striae on the 

 ■surface of the cast, it is nearly straight, or at the most, only 

 gently convex in front of the beaks, and nearly parallel with the 

 length of the shell, sloping slightly downwards. Behind the 

 beaks it is gently convex, nearly straight, and slopes downwards 

 to the narrowly rounded posterior angle, the latter situated at 

 about oue-third the height of the shell. The margin behind 

 the beaks is compressed. Close under the beaks, in front, 

 there appears to have been a short escutcheon. From the um- 

 bones backwards for about six lines, a linear groove runs along 

 close to the dorsal edge on each side. This may be related to 

 tlie ligament. 



The most projecting point of the anterior extremity appears 

 to be situated considerably above the mid-height of the shell, 

 near the hinge line. The posterior angle is below the mid- 

 heiirht. 



Surface concentrically striated. 



Length 3 inches; greatest height, a little in front of the mid- 

 length 18 lines; greatest depth of both valves, just below the 

 iumbones 8 lines. 



The specimen was collected by Sir W. E. Logan in the Upper 

 Silurian rocks at Port Daniel on the Bay of Chaleurs. 



Genus Pteronitella, (n. gen.) 



Among the fossils collected at Arisaig, Nova Scotia, in the 

 •Upper Silurian, there are many casts of the interior, of several 

 species congeneric with Avicula retrojiexa (Hisinger). These 

 show thot in front of the beaks, there are several small cardinal 

 teeth, and that close beneath the hinge line there are several 

 more or less elongated posterior teeth. This arrangement is 

 quite different from that of both Avicula and Fterinea. to which 

 these shells are usually referred. There is a strong anterior 

 muscular impression and the whole structure of the hinge re- 

 sembles closely that of Cyrtodonta. 



Prof. McCoy has noticed the teeth, in his description of 



P. retrojiexa (Pal. Foss., p. 262) but does not seem to think 



their structure of generic importance. The above generic name 



is proposed, to include P. retrojiexa and some others, soon to be 



■described. 



