BRACHIOPODA. 241 



Description — Shell large, sub-oval in outline, strongly convex, length usually greater 

 than the width. A medium sized brachial valve measured 53 mm. in length and 40, 

 mm. in width. The dimensions of the pedicle valve of another individual are: Length 

 45 mm., width 36 mm., the greatest width anterior to the middle of the shell. The 

 lateral margins of both valves are conspicuously flattened. 



Pedicle valve depressed convex, the greatest convexity just below the umbonal 

 region. Near the anterior margin a broad shallow sinus is developed. Whether this is 

 extended into a linguiform projection as described by Hall cannot be ascertained from 

 the imperfect material at hand. The sides of the sinus curve upwards forming sharp 

 lateral margins, the sides of which slope abruptly to meet the opposite valve. Beak 

 pointed and only slightly incurved. 



Brachial valve convex to gibbous. The shell surface slopes uniformly from the 

 middle towards the lateral margins, then turns abruptly, meeting the other valve at 

 right angles. Beak incurved more than that of the pedicle valve. 



Surface of each valve marked with about 40 radiating, angular plications, and several 

 indistinct ones near the postero-lateral margins. Intervening spaces narrow. 



Remarks — A limited number of separate valves have come under observation. 

 They are distinguished from the other species of Plethorhyncha with which they are 

 associated by their larger size, and their coarser plications. With the exception of one 

 fragment none of the specimens attain the proportions of the larger New York speci- 

 mens. 



Horizon — Lower beds. 



Figured specimens No. 27461 Walker Museum. 



Plethorhyncha principalis (Hall) 



Plate 64, figures 1-3. 



1856. Rhynchonella principalis Hall, Regents Rept., p. 84, Paleozoic Fossils, 1857, 



p. 44. 

 1859. Rhynchonella principalis Hall, Nat. Hist. N. Y. Pal., vol. 3, p. 443, pi. 106, fig. 

 4. 



Description — Shell large, elongate oval, relative length and width variable. The 

 dimensions of an average sized pedicle valve are: Length 45 mm., width 38 mm., 

 the greatest width anterior to the middle of the shell. 



Pedicle valve depressed convex, arched in the umbonal region, in some specimens 

 forming a broad median ridge, the surface gradually flattening out toward the anterior 

 margin to form a broad, shallow sinus. The shell surface slopes uniformly from the 

 middle and forms sharp, lateral margins as it becomes abruptly inflected to produce 

 flat, lateral areas. Beak flat, acute, slightly incurved. 



Brachial valve convex to gibbous, flattening somewhat towards the anterior 

 margin. Lateral margins abruptly incurved to meet the opposite valve at right angles. 

 Beak arched and incurved more strongly than the beak of the pedicle valve. 



Surface of each valve marked with from 60 to 80 radiating plications, which are 

 exceedingly fine near the beak. . These are crossed by faint, concentric growth lines. 



Remarks — This species occurs abundantly in the Little Saline limestone, and is 

 commonly represented by somewhat fragmental separate valves. This species is 

 closely allied to Plethoryncha barrandii Hall, and in many cases it is almost impossible 

 to draw a line of demarcation between the two species, one grading imperceptibly into 

 the other. In general, however, this species may be distinguished by its smaller size, 

 by the finer plications, and by the proportionately broader brachial valve. 



Horizon — Lower and upper beds. 



Figured specimens No. 27460 Walker Museum. 



