DESCRIPTION OF SPECIES BRACHIOPODA 297 



hinge line, the beak more strongly incurved than that of the opposite 

 valve, the cardinal area about one-half the width of that of the pedicle 

 valve, arched and directed posteriorly and toward the beak of the- pedicle 

 valve. Surface of both valves marked by small, rounded, radiating costse, 

 about three of which occupy the space of one millimeter; the costse are 

 separated by rounded furrows about equal in width to the costae them- 

 selves; upon the pedicle valve the costae increase by bifurcation, and on 

 the brachial valve by implantation ; the surface is also marked by more or 

 less crowded, subimbricating lines of growth. 



The dimensions of a rather large individual are: Length, 30 milli- 

 meters; width, 36.5 millimeters; thickness, 23 millimeters; length of 

 hinge line, 20 millimeters ; height of cardinal area, 4 millimeters. 



Remarks. — This species in the Fern Glen fauna does not attain so 

 large a size as it does in the Burlington limestone, where examples not 

 infrequently exceed 50 millimeters in width, but in all other respects the 

 Fern Glen examples are essentially identical with specimens from the 

 higher formation. The species should probably not be considered as 

 distinct from the European S. resupinata of the mountain limestone, but 

 until a careful comparative study of the American and European forms 

 can be made it is perhaps not advisable to drop the present name. The 

 species is a close ally of the Devonian S. striatula; it differs from that 

 species especially in the larger size to which it grows and in the much 

 less development of the mesial sinus of the pedicle valve. 



CHONETES ILLINOISENSIS Worthen 



Plate 12, figure 11. 



1858. Chonetes logani Hall, Geological Survey of Iowa, volume 1, part 2, page 

 589, plate 12, figures 1-2 (not G. logani N. & P., 1855). 



1SG0. Chonetes illinoisensis Worthen, Transactions of the Saint Louis Academy 

 of Science, volume 1, page 571. 



18G8. Chonetes illinoisensis M. & W., Geological Survey of Illinois, volume 3, 

 page 505, plate 15, figure 8. 



Description. — Shell concavo-convex, the length from .6 to .7 of the 

 width, hinge line usually a little shorter than the total width of the shell, 

 but sometimes equaling the width. Lateral margins of the shell nearly 

 straight or slightly convex posteriorly, directed at nearly a right angle 

 to the cardinal line, more strongly curved anteriorly and passing with a 

 regular arcuate curvature into the anterior margin, which is often nearly 

 straight for a short distance in its central part. Pedicle valve depressed 

 convex, somewhat compressed toward the cardinal extremities, flattened 

 along the mesial region and often depressed in a shallow, ill defined 



XXVI — Bull. Geol. Soc. Am., Vol. 20, 1908 



