FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 35 



In interior of ventral valve the vertical dental plates form the walls of 

 the fissure, and extend from beak to bottom of shell ; between these a small 

 rounded mesial ridge divides the 'muscular scars, which extend over two elon- 

 gated depressions, margined on their outer side by the prolonged bases of the 

 dental plates. The cardinal muscles appear to have occupied greater portion 

 of anterior division of these two depressions ; the pedicle muscles occupying 

 the external and posterior part of same space. 



The adductor was probably attached to each side and close to mesial ridge. 

 In the socket valve the fissure is partly or entirely occupied by a more or 

 less produced simple shelly process, to which were affixed the cardinal muscular 

 fibres, the inner socket walls are considerably prolonged into cavity of the 

 shell, under shape of projecting laminae, to the extremity of which free, fleshy, 

 spiral arms may, perhaps, have been affixed. Under this shelly process a 

 longitudinal ridge separates the quadruple impressions of the adductor, which 

 on each side forms two deep oval depressions, placed obliquely one above the 

 other, and separated by lateral ridges, branching from the center one. The 

 genus Orthis is very nearly related to Strophomena, from which the more 

 typical forms may in general be easily separated by their usually greater con- 

 vexity, and the rounder and shorter hinge-line. 



Orthis biforata. Schiotheim. 



Plate XXIX., figures 18 to 22. 



Terebratulites biforatus, Schiotheim, Petrefact. P. 265 1820 

 Orthis biforata, Meek. Pal. of Ohio, Vol. I., p. 1121873. 

 See the list of synonyms in last cited Report. 



Shell small, transversely oval-subquadrate, moderately convex, wider than 

 long ; hinge-line a little less than the greatest width of the shell ; cardinal 

 angles obtuse, seldom rounded. Surface plicate. 



Ventral valve depressed convex ; mesial sinus well defined from beak to 

 front, deep, with flattened bottom, containing four plications, the central ones 

 starting from the beak, while the lateral start about the middle of valve from 

 marginal slopes of the sinus. 



In one of my specimens one of the lateral plications within the sinus is 

 wanting; beak a little more elevated, and somewhat stronger than that of 

 the opposite valve, only faintly curved, almost straight ; cardinal area narrow, 

 but increasing in height towards beak ; foramen almost an equilateral tri- 

 angle. 



Dorsal valve generally a little more convex than other, greatest convexity 

 at the umbo, from where it curves gently and regularly to lateral and front 

 margins. 



