44 G. F. MATTHEW: ILLUSTEATIONS OF 



near or at the hinge line, which is not produced; moderately convex, highest in the pos- 

 terior third, flattened toward the front and sides. Umbones not prominent. 



The dorsal valve rises rapidly from the hinge line and has but a narrow, flattened 

 space at the angles. It is about one third as high as it is long. The hinge plate has a 

 narrow area, which is striated lengthwise, and bears two tooth-like processes close to the 

 umbo, the point of which is bent down to the hinge line. 



The ventral valve is somewhat more elevated than the dorsal. Hinge area triangular, 

 sloping backward to the umbo, and bearing fine striœ parallel to the hinge line. Umbo 

 elevated above the hinge line to a height fully equal to one third of the length of the 

 valve. Foramen large, truncate-pyramidal in outline. 



Surface of the valves ornamented by about twenty rounded plica-, radiating (in the 

 ventral valve) from the edge of the hinge area, opposite the foramen, and not from the beak 

 alone ; those on the middle fifth of each valve are crowded together, those outside of 

 these ai'e more prominent, and are continuous from the umbo ; those toward the hinge-line 

 are faintly marked and widely separated. The radiating plicae do not (or rarely) increase 

 by bifurcation. Both valves bear numerous concentric striae, and are also marked by a few 

 distinct squamose lines of growth. 



The mould or cast of the interior of the valves of this species, is nearly .smooth, or is 

 marked by faint radiating striie ; the margin of the mould, however, often exhil)its a crenu- 

 lated appearance corresponding to the plicae of the outer surface. There is a wide, smooth 

 median depression at the top of the mould of the dorsal valve, near the umbo. 



This little species is well separated from Orthis Billingsi by many differences which 

 become ob^âous on a comparison of the two species : by the smooth spot in front of the 

 umbo of the dorsal valve, by the less numerous and straighter radiating plicae, by the 

 fulness of the A^alves toward the cardinal angles, by the smooth mould of the interior, etc. 



Among the Orthides of the "Welsh Cambrian strata, O. Carausii resembles this species, 

 but it has a few more plicœ, a flatter dorsal valve, and a smaller foramen proportion- 

 ately ; it is also foirr times as large as the Acadian species. Of the Swedish species, 

 O. exporrecta (Linnarsson) comes nearest our species, but differs in its larger size, flat dorsal 

 valve, plicated mould, etc. All the Swedish species are from the upper part of the Para- 

 doxidian zone, whereas the Acadian forms reach down to the oldest beds in which Para- 

 doxides is known. The Orthides of Div. l.c of the St. John group and Kutorgina 

 Latourensis are remarkable for the crowding together of the plicae or striae on the median 

 fifth of the valve, and in this remind us of the arrangement of the ribs on the surface of 

 the A'alves of the genera, Spirifer and Airi/pa ; a flattened area is thus formed across the 

 middle of the valve. This feature is less obvious in O. Billingsi than in the other two 

 species. 



Length of the A^alve in O. Quacoensis, 5 mm. Width, 8 mm. 



Horizon and Locality. In the shales of Div. l.c, at Portland and at Porter's Brook, St. 

 Martin's. Scarce. 



