DESCRIPTION OF SPECIES 817 



Brachial valvo less coinc.x than the pedicle; Jiiolu^st at the front of the 

 fold ; eaeh lateral slope marked by (> to 7 j'oiinded plications. Mesial fold 

 sharply defined, rounded, much elevated in front. Cardinal area very 

 narrow and nearly at right angles to that of the pedicle valve. Beak 

 small, incurved. Lines of growth not imbricating. 



This is the only species of Spiriferina found in the fauna. It differs 

 from S. salemensis in the greatest width being in front of the hinge line, 

 the cardinal extremities being rounded, cardinal area much lower and 

 narrower, mesial sinus rounded, delthyrium wider compared to height, 

 usually one or two more plications on each side of a valve, median septum 

 longer and mesial fold rounded. It differs from S. spinosa in lacking the 

 spines, in the median septum ])eing larger, and the cardinal extremities 

 never being angular. 



SPIRIFEIi I'ELL^NtilS (Weller) 



Plate 18, figures 7-9 



1914. Spirifer pelloensis Weller, Illinois State Geological Survey, monograph 1, 

 pages 340-341, plate 45, figures 1-31. 



This species is abundant and has a wide variety of shapes and sizes. 

 The number of plications is fairly constant, varying from 18 to 22 on the 

 pedicle valve in 25 specimens chosen at random. Usually there is one 

 well developed plication in the sinus and two weaker plications formed 

 by the bifurcation of the marginal plications. Some specimens show 

 three well developed plications in the sinus and two v^eaker plications 

 formed as mentioned above. Most of the forms from the Amsden agree 

 with Weller's description, but there is one distinct variation in which the 

 hinge line is constantly longer than the shell, and the fold is much more 

 elevated than in the type. 



Measurements of eight typical specimens show an average length of 

 19.9 millimeters; breadth, 26.5 millimeters; thickness, 12.7 millimeters. 

 Measurements of four of the other variety show average length, 20 milli- 

 meters; breadth, 36 millimeters; thickness, 16.5 millimeters. 



The typical forms are much the more abundant in the fauna and the 

 larger variety is relatively rare. 



SPIRIFER WELLER I n. sp. 



Plate 18, figures 10, 11, and 16 



Shell below medium size ; length and breadth subequal ; greatest breadth 

 in front of the hinge line; cardinal extremities rounded. The average 

 size of eight full-grown specimens is: I^ength, 16 millimeters; breadth, 

 17.6 millimeters; thickness, 10.4 millimeters. 



