728 CAMBKIAN BRACHIOPODA. 



General form subquadrate to transversely semioval, with the cardinal extremities subacute 

 to obtusely angular. Hinge line straight, usually equal to or greater in length than the width 

 of the body of the shell. Shell substance dense, granular, with minute pores arranged in scat- 

 tered radiating Imes; none of the pores appear to pass through more than one lamella of the 

 shell. 



Surface Avith narrow, rounded, radiating ribs, that increase by both bifurcation and inter- 

 polation; the ribs are roughened by concentric lines and ridges of growth that arch about the 

 base of strong, acute spines; the spines are located on the ribs, usually just back of a ridge of 

 growth; they are in more or less irregular, concentric rows toward the front of the shell, but 

 on the central portions they may be scattered without any system of arrangement; each spine 

 is larger at the base, tapering rapidly, and curving gradually backward at about one-half its 

 length. 



The ventral valve is more or less elevated at the umbo and apex; in some examples it is 

 subpyramidal (PI. C, fig. le), where the height is to the length as 3 to 6, and width 9 mm.; in 

 other shells the proportion is 4 to 11, and width 17 mm.; the slopes from the apex to the mar- 

 gins are nearly straight or slightly convex; the form of the umbo and apex varies from a broad, 

 rounded umbo terminating in a minute beak curving over the area, to an erect, vertical, more 

 or less rounded apex, with a broad base rising abruptly from the elevated umbo; a shallow, 

 rounded, median sinus occurs on most shells, but it is sometimes absent. Area high, and crossed 

 by transverse hues of growth ; it is usually inclined over the hinge line, but it may be vertical 

 or inchned forward (fig. le) ; it is divided by a strong delthyrium, which is covered by a convex 

 deltidium that the author of the species, Bilhngs, describes [1861c, p. 10] as perforate at the 

 apex.'' I have not been able to verify this in a large collection of material from various local- 

 ities; casts of the interior of the extended beak indicate but do not prove that there was a 

 perforation; the front edge of the deltidium is arched so as to leave a space of variable height 

 between it and the plane of the hmge line; in three examples the area is divided into three 

 parts by longitudinal lines (PI. C, fig. le"), two of the fines bound the delthyrium, and one 

 on each side corresponds in position to the "flexure" lines in Obolus and Hipparionyx. A 

 cast of the interior of a low ventral valve (PL C, fig. If) shows a broad delthyrium, strong teeth, 

 and supporting dental plates which are produced on the inside so as to form a short, elevated 

 base (pseudospondylium), and on the outside the plates are continued partly about the space 

 occupied by the points of attachment of the diductor muscles. 



The dorsal valve is moderately convex at the umbo, sloping gently from there to the 

 margins; usually the slope is convex, but in. one example it is slightly concave; area narrow 

 and vertical or slightly inchned over the hinge line ; casts of the interior show the crura, points 

 of attachment of posterior adductor scars (e, PI. C, fig. Ig; see fig. 3a), area of attachment of 

 diductor scars (d', PL C, figs. Ig, li). Vascular and ovarian markings unknown. 



Ohservations. — This shell has a wide geographic distribution. I have collected it at the 

 type locality in the townsliip of Georgia, Vermont; at Bic on the lower St. Lawrence Eiver; and 

 near York, Pennsylvania. The matrix at Bic is a finely granular, shghtly arenaceous lime- 

 stone in which the outer form of the shell is well preserved, but the spines are rarely seen. At 

 Swanton and Georgia, Vermont, the shell occurs in siliceous limestone and arenaceous shale, 

 and a cast of the outer surface shows the spines ; the material from the finely arenaceous lime- 

 stone at the Emigsville locahties, discovered by A. Wanner, of York, Pennsylvania, is the best 

 preserved and afFords excellent casts of the interior and exterior of the valves. In all of the 

 localities the variation in the form and elevation of the ventral valve occurs; specimens of the 

 ventral valve from Bic, Canada, show a shorter deltidium than is usually present. A number 

 of exfoliated shells occur in the collection from the dark-gray compact limestones of eastern 

 New York and are doubtfully referred to this species. 



a Through the courtesy of Dr. J. F. Whlteaves, of the Geological Survey of Canada, I had the opportunity of examining the types of "Orthisina 

 festinata." None of them preserve the apex of the deltidium, so it is impossible to determine upon what Billings based his statement that the 

 deltidium was perforate. 



