84 



On the umbo of zhe ventral valve, a mesial sinus commences, which 

 gradually increases in width to the front margin ; more than half of which 

 is affected by it. The dorsal valve has a mesial elevation, on each side of 

 which there is a sinus of just sufficient strength to induce the idea of a 

 trilobed surface. The area of this valve is incurved over that of the 

 ventral valve and almost entirely conceals it, at the same time completely 

 closing the foramen. The socket plates are exceedingly small, no traces 

 of them being seen in a cast of the interior of the umbo, which is perfect 

 to within one line from the edge of the ventral area. 



The surface is covered with strong, rounded, radiating ribs, increasing 

 in number both by bifurcation and intercalation. The width of the fully 

 developed ribs is about one line, but in some places on the surface, 

 where the ribs are newly divided, there are often to be seen four or five of 

 them in the width of two lines. The shell is subject to interruptions of 

 growth, and in such cases the form of its younger stages is very dis- 

 tinctly represented on the surface. 



The front margin is sometimes uniformly rounded, often with a sinus in 

 the middle, as in the specimen figured, while others exhibit an obscure 

 projecting lobe in place of the sinus. 



The length of large specimens is about four inches; width, three inches 

 and a half ; depth of both valves, two and a half inches. 



Locality and Formation. — L' Anse a la Vieille on the Bay of Chaleurs. 

 Middle Silurian. 



Collector. — Sir W. E. Logan. 



Stricklanbinia brevis. (Billings.) 1859. 



Plate 6, figs. 2, 2a, 2b, 2c. 

 Stricklandinia brevis. (Billings.) Canadian Nat. and Geol. vol. 4 p. 135. 1859. 



Bescription — Transversely sub-ovate or sub-pentagonal ; hinge line 

 straight, equal to about two-thirds the greatest width ; cardinal angles 

 rounded ; greatest width a little above the mid-length, in front of which 

 the sides converge with a gentle curve to the middle of the front margin. 

 Both valves moderately and about equally convex, most prominent at about 

 one-third the length from the beak. Ventral valve with a concave mesial 

 sinus, which commencing at the beak gradually widens to nearly one-third 

 the whole width of the shell. The sides and cardinal angles somewhat 

 compressed. Umbo small ; beak closely incurved, touching the dorsal valve ; 

 area concealed. Dorsal valve with a rather strong mesial fold which, at 

 the front margin, is about one-third the whole width of the shell ; on each 

 side of the fold, a slight longitudinal depression. The beak seems not to 

 overhang the hinge line, but is slightly elevated in the plane of the lateral 



