DESCRIPTIONS OF SPECIES. 



65 



cuneiforme; the ribs of Flabellum sedecimcostatum are somewhat weaker and more of 

 a size (however, my specimens are more worn). Therefore, it is my opinion that, 

 although Flabellum scdeHmcotitatum is very near Flabellum cuneiforme, still there is 

 more of a basis for not identifying these two species. 



The comparison is made in S2:)eakiug of the relations between TrocJio- 

 cyathus discoides Sokolow and TrochocyatJms lunulitiformis (Conrad), (see 

 page 94 of this paper). The amount of compression of F. cuneiforme var. 

 wailesi varies much, so that apparently the only basis for separating the two 

 species lies in the character of the costse. Whether or not this will hold, 

 must be determined by further work. 



Flabellum cuneiforme var. magnocostatum var. iiov. 

 PI. IV, figs. 4 and 4a. 



In the Red Bluff beds, exposed at Garlands Creek, Mississippi, a curious 

 variety of Flabellum cuneiforme is found. This variety is charactei'ized by 

 having a thick wall and four or five very prominent costse on the faces. It 

 grades into forms without prominent costse and with thin- walls, j^assing 

 directly into var. wailesi. 



These varieties of Flahellum cuneiforme occur in a definite geologic suc- 

 cession, and were it not for the presence of intermediate forms, we should 

 be obliged to consider them distinct species. They are a series of connected 

 forms, and we may confidently represent their genetic relations by the 

 following diagram : 



Red Bluff. 



Jacksonian. 



Upper Claiborne. 



Lower Claiborne. 



MON XXXIX 5 



Vai'. wailesi. 



Var. magnocostatum. 



Var. wailesii (and probably 

 typical cuneiforme). 



cuneiforme (probably 

 typical). 



Var. fragile. 



Var. acutiforme. 



Var. pachyphyllum. 



