CANADIAN FOSSILS. 41 



area oblique, broad-triangular (about 140°) ; foramen almost linear, 

 continued quite to the apex. 



Surface of both valves closely radiated by about thirty convex, 

 rod-like ribs, with narrower interstices, the ribs quite simple in the 

 ventral valve, but interlined in the upper or dorsal valve by fine 

 regular threads. All are crossed by delicate, inconspicuous striae of 

 growth. Old specimens are antiquated near the margin. The shell is 

 thin ; the interior is deeply scored, for some distance from the margin, 

 by conspicuous furrows between the ribs ; each rib is impressed by 

 a median groove. 



The space for the muscular impressions in the ventral valve 

 (fig. 3a), is moderately large, rhomboidal, bilobed at the base, cir- 

 cumscribed by external blunt ridges, and divided by a very slight 

 median one. The hinge-plates are very short, and diverge at about 

 80°. The ovarian spaces (b) are well marked. 



The interior of the dorsal valve (fig. 4), with the muscular impres- 

 sions very strongly marked (b, c), shews them divided by a broad, 

 rounded ridge. The brachial processes or hinge-teeth (a) are very 

 short, diverging at 50°, and with a linear central tooth (cardinal 

 process), which bisects the narrow foramen, and bends backward 

 with the area. The ribs shew very strongly in the interior, fully 

 half-way up the shell. 



The area in the dorsal valve is flat, and forms an angle of 50° with 

 the valve itself: that of the ventral valve is inclined at about 70° ; 

 it is very little curved. The two areae meet at about 120° in the 

 young shell, and about 90° to 100° in the adult. 



A great advance was made in the classification of the Brachiopoda 

 by Dalman, when he established the genera Orthis and Leptaena for 

 the numerous Silurian species he was acquainted with. And little 

 has since been done for the family of the Orthida, except the sub- 

 division of the former genus, and the separation from both of the 

 flat, semicircular forms now known under the name of Strophomena. 

 The three genera are clearly separable, if account be taken of the 

 general habit, as well as of the characters of the hinge and muscular 

 impressions, for while Leptcena has the valves involute, and the 

 muscular impressions of the entering valve greatly elongated, 

 Strophomena and Orthis have these four impressions subquadrate and 

 arranged in a circumscribed group, Strophomena being further distin- 

 guished from Orthis no less by its expanded flat form, than by the 

 large bilobed cardinal process between the teeth in the dorsal valve. 

 In Orthis the process is simple, often linear, and the two teeth 



