BRACHIOPODA. 443 



Ortlils (D.ihiKiiielhi) u-.stu<liiiari:i.] 



than twice the size], suboval in form and extending to near the middle of the valve; 

 cardinal process very small [comparatively large, striated] and trifld; brachial pro- 

 gesses comparatively rather stout and prominent; internal surface having the radi- 

 ating stria? of the exterior rather distinctly impressed through, as it were [each 

 with a central furrow], in consequence of the thinness of the shell, ami finely 

 granular, the granules being apparently connected with the punctate structure of 

 the shell. 



" Ventral valve compressed-convex, the greatest convexity being near, or a little 

 behind, the middle, along a more or less prominent, undefined ridge that sometimes, 

 but not always, imparts a subcarinate appearance to the central and umbonal 

 regions; beak small, projecting somewhat beyond that of the other valve, abruptly 

 pointed and rather distinctly arched, but not strongly incurved; area about twice as 

 high as that of the other valve and with its sharply defined edges sloping to the 

 lateral extremities of the hinge, directed and arched obliquely backward with the 

 beak; foramen [delthyrium] having nearly the form of an equilateral triangle, but 

 rather narrowed upward to the apex of the beak [with a distinct linear ridge along 

 each wall] and partly occupied by the cardinal process of the other valve. Interior 

 showing the teeth to be moderately prominent; concavity for the muscular impres- 

 .sions very shallow, somewhat bifid anteriorly and not defined by a very distinct 

 marginal ridge; at the base of the dental plates are situated the narrow adjuster 

 scars, terminating at the transversely striated pedicle muscle, which occupies the 

 posterior portion of the rostral cavity; on the inside of the former are the large 

 diductors separated by the small, elongate adductors; striae and fine granules of the 

 interior as in the other valve. 



"Surface of both valves ornamented by numerous, distinct radiating striae that 

 usually bifurcate about three times between the beak and free margins; posterior 

 lateral striae so strongly curved that a part of them run out on the hinge-line. 

 Numerous very minute, regularly disposed, concentric lines may also be seen by 

 the aid of a magnifier, most distinctly defined in the furrows between the radiating 

 striae, while a few distinct, subimbricating, stronger marks of growth are usually 

 seen in adult shells." Shell structure distinctly punctate. 



Size of shell, outline, convexity of valves and number of striae are more or less 

 variable features at all localities of this species. "The shells of this species usually 

 characterize the Trenton and Hudson River beds at nearly all their outcrops, being 

 one of the most persistent, as well as most characteristic fossils of these layers. 

 Still, there are so many varieties of it that it is often difficult to refer it, without 

 some hesitation, to its original place. Very many of these varieties have been 

 described as distinct species, and others as varieties under varietal names, until 



