ICHTHYOCRINIDAE 32I 



larger externally than on the inner floor of the calyx. This feature is well illustrated by 

 figures 1 ic, d, of Plate XXXIX, which are exterior and interior views of the same specimen, 

 and by figure 12, a vertical section showing the dorso-ventral course of the sutures of the 

 various plates from infrabasals up. The arcuate sutures are very conspicuous in all the 

 species, especially those from the Knobstone, in which they have the appearance of imbrica- 

 tion so prominent in some Ichthyocrinus. 



Metichthyocrinus is a rare genus, with a very limited stratigraphic range ; but few species 

 are known, and it has thus far been confined to the Kinderhook, Knobstone and Burlington 

 formations. This statement is in apparent conflict with the record as to the horizon of certain 

 species. M. clarkensis, from Clark County, Indiana, is stated by the authors to be from the 

 Keokuk or Warsaw. The type specimen was purchased by Mr. Gurley from a dealer, as 

 were many of Miller and Gurley's types, and there is no authentic record of its exact locality. 

 It is supposed to have been found near Henryville, where the fossiliferous exposures belong 

 to the Knobstone group, which is the source of all other known specimens. The type : of 

 Troost's M. tiaraeformis is from White's Creek Springs, Tennessee, and is also credited to 

 the Keokuk ; but from the appearance of this and some other specimens of the same species 

 found at that locality, and from its known occurrence in some of the Knobs south of Louis- 

 ville, Kentucky, there is no doubt that it comes from the lower beds at White's Creek, and is 

 therefore from the Knobstone, or New Providence shale, approximately equivalent to the 

 Lower Burlington (see Proc. U. S. Nat. Mus., vol. 41, 1911, pp. 175-208). No trace of the 

 genus has been found at any of the typical Keokuk localities, and no authentic- specimen in 

 the Upper Burlington. 



The points of differentiation of the species in this genus are few, and are mostly such as 

 might shade into one another by easy transitions among forms prolific in individuals. The 

 general form and proportions of the crown count for something, and the number of secundi- 

 brachs, relative breadth of base, and imbrication of the plates, are useful characters. I have 

 arranged the species as follows : 



The Species of Metichthyocrinus 



I. IIBr usually 3 ; plates not imbricated. 



Crown ovoid, elongate M. burlingtonensis. 



II. IIBr usually 4 ; plates more or less imbricated. 



Crown ovoid, elongate M. clarkensis. 



Crown globose. 

 Base broad M. tiaraeformis. 



