294 S. WELLER FAUNA OF THE FERN GLEN FORMATION 



ORTHOTHETES RUBRA n. sp. 



Plate 12, figures 4-5 



Description. — Shell of medium size when full grown, wider than long, 

 the hinge line straight and slightly shorter than the greatest width of the 

 shell ; lateral margins slightly convex posteriorly, becoming more strongly 

 curved in front and rounding regularly into the anterior margin, which 

 is nearly straight in the middle half of the shell; shell substance rather 

 thick. Pedicle valve depressed convex, most prominent toward the beak, 

 the surface sloping gently toward the front and a little more steeply 

 toward the cardinal extremities with a slightly convex curve; cardinal 

 area rather low, apparently flat, but not well shown in the type specimen. 

 Brachial valve about as convex as the pedicle, slightly compressed toward 

 the cardinal extremities. Surface of both valves marked by fine radiating 

 costa?, some of which are slightly larger than the others; the costae in- 

 crease by intercalation and about three occupy the space of one milli- 

 meter. The surface is also marked by fine concentric lines, which are 

 apparently more conspicuous toward the cardinal extremities, on which 

 parts of the shell the radiating costre appear to be minutely serrate by 

 reason of the crossing of the concentric lines. A few more or less incon- 

 spicuous lines of growth occur upon the main body of the shell, but in 

 old examples the growth lines become much crowded toward the margin 

 and the shell thickened. 



The dimensions of a nearly complete example are: Length, 25 milli- 

 meters; width, 36 millimeters; length of hinge line, 32 millimeters; 

 thickness, estimated, 10 millimeters. 



Re marls. — This species is rather rare in the fauna. It resembles 

 0. lens (White), from the Louisiana limestone, but it grows to a much 

 larger size, is proportionally broader, and is marked by somewhat finer 

 radiating costse than that species. Some of the smaller and immature 

 examples are of about the average size of the full grown specimens of 

 0. lens, but they may be distinguished from that species by their form 

 and costse. 



ORTHOTHETES ? sp. 



A single imperfect internal cast of the brachial valve of a large Ortho- 

 thetes or Derbya-like shell has been observed in the Fern Glen fauna 

 which is too incomplete for identification. When complete the shell 

 must have had a length of 40 millimeters and a width of about 55 milli- 

 meters. It has a broad and strong cardinal process and was marked by 

 rather strong concentric wrinkles. 



