158 MISSISSIPP1AN BRACHIOPODA 



muscular scars; muscular scars rather faint, their anterior margin not 

 reaching to the middle of the valve, obscurely quadripartite ; the lateral 

 and anterior margins crenulate anteriorly. 



Surface of both valves marked by fine, depressed, radiating costas, 

 about four or five of which occupy one millimeter along the front margin 

 of an average size specimen. The costse increase by bifurcation and inter- 

 calation and remain nearly uniform in size. Crossing the costae upon well 

 preserved shells there are exceedingly fine concentric lines, and stronger, 

 more or less irregularly distributed concentric lines of growth. The shell 

 substance is pierced by radiating tubules which follow the direction of 

 the costffi, with openings at intervals along the summits of the costae. 

 Finer tubules, such as are present in E. burlingtonensis, have not been 

 observed, although they may be present. 



Remarks. This species occurs in great numbers in the Fern Glen Kin- 

 derhook of Illinois and Missouri. It is characterized by its exceedingly 

 fine costag, by the distinct mesial sinus of the brachial valve, by the po- 

 sition of the cardinal area of the pedicle valve which slopes posteriorly 

 more nearly in the plane of the valve than in any other of the species here 

 described, and in the small size of the cardinal process. By one or more 

 of the above characters the species may be distinguished from any of the 

 other species. 



In its typical expression this species is a small form, but associated with 

 it are a sm'aller number of a much larger shell approaching R. burlington- 

 ensis in size and form. The smaller specimens have usually been con- 

 sidered as being the immature examples of the larger form, but many of 

 the small individuals give every evidence of being fully mature, and it is 

 more than probable that two species are represented. The larger one 

 does not exhibit the distinct mesial sinus of the brachial valve which is 

 such a marked characteristic of the smaller one, the cardinal area of the 

 pedicle valve does not lie quite so near the plane of the valve, the cardinal 

 process is more prominent and the muscular impressions are somewhat 

 larger. Most of these characters, however, except the sinus of the brachial 

 valve which ought to persist, might be due to the greater age of the larger 

 shells. 



Horizon. Fern Glen formation. 



RHIPIDOMELLA TENUICOSTATA n. sp. 

 Plate XX, Figs. 27-29 



Description. Shell small, lenticular in form, subovate in outline, vary- 

 ing from longer than wide to wider than long, the greatest width anterior 

 to the middle, the hinge-line short, usually not exceeding one-third the 

 width of the shell, the cardinal extremities rounded. The dimensions of 

 two nearly perfect specimens are: length of pedicle valve 15.3 mm. and 



