141 



It is often very obscurely marked. The position of the point where it 

 reaches the margin, is sometimes situated near the mid height of the pos. 

 terior end of the shell, but is usually lower down ; often higher up in one 

 valve than in the other. 



The surface is covered with concentric ridges from half a line to two 

 lines wide. There are, also, very fine strii^e, four or five in one line, on 

 and between the larger. 



Length of a large specimen twenty-seven lines ; height at the umbones 

 fourteen lines, depth of the two valves in connection fourteen lines. 



This is, probably, the species that has been referred to G-. cingulata') 

 from which it diflfers in having the groove terminating near or on the lower 

 half of the posterior extremity, instead of in the middle of the ventral 

 margin. 



In many of the specimens, the anterior margin slopes obliquely back- 

 wards and downwards, so that, when the fossil is placed upright resting 

 on the middle of the ventral margin, the beaks constitute the most pro. 

 jecting point of the anterior end. The hinge-line is also often so much 

 elevated that its posterior extremity is near the mid-length of the shell, 

 and forms there a prominent, obtuse angle. The form is then more or 

 less rhomboid-ovate instead of oblong-ovate. In front of the groove an 

 obscure concave depression, in the side of the shell, commences and gra- 

 dually widening extends to the base. This is always present, but is often 

 barely perceptible. 



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

 Collector.— SI. C. AVeston. 



Genus Pteronitella. (N. Gen.) 



Among the fossils collected at Arisaig, there are many casts of the 

 interior of several species congeneric with Avicula retroflexa (Hisinger.) 

 These show that, in front of the beaks, there are several small, anterior 

 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 Pterinea, to which these shells are usually 

 referred. There is a strong anterior muscular impression, and the whole 

 structure of the hinge resembles closely that of Cyrtodonta. 



Prof. McCoy has noticed these teeth in his description of P. retroflexa 

 (Pall. Foss., p. 262), but did not seem to think their structure of generic 

 importance. I propose to separate F. retroflexa and others of similar 

 structure, from Fterinea, and to place them in a new genus : — 

 Fteronitella. 



