1887.] NATURAL SCIENCES OF PHILADELPHIA. 83 



culty; but it remains to be seen how far the authors have succeeded 

 in proving it. 



A covering of the summit openings in various genera has been 

 described by several writers. This has been generally considered 

 as representing the same structure in all these genera ; whereas there 

 is to be found among the coverings, thus described, two distinct 

 structures, which are totally different from each other, and are 

 characteristic, so far as observed, of distinct groups of the Blastoidea. 



The first of these of which any detailed account has been 

 attempted, was observed by Roemer in 1851, in Elaeacrinus 

 Verneuili,^ which he described as having the summit plates closed 

 by a hexagonal central plate, surrounded by six others, four of 

 equal size and two smaller. Shortly afterwards Shumard/ in 

 describing his new species Pentremites Sayi, stated that "the central 

 opening is closed by minute, usually pentagonal and hexagonal 

 plates, arranged in a manner somewhat similar to those of Pentremites 

 (Elaeacrinus) Ver)ieuill," and he added in a note: "the same 

 structure occurs in Pentremites Norwoodi and P. melo Owen and 

 Shumard, of which I have fully satisfied myself from an attentive 

 examination of many specimens." 



In 1863, Dr. White, in a jjaper on the summit structure of 

 Pentremites, ^ confirmed the observations of Shumard as to Pentremites 

 Norivoodi, and stated that in this species the whole central space 

 between the summit tubes and the anal aperture "is overlaid with 

 an integument of microscopic plates, entirely covering the central 

 aperture, passing out between the bases of the tubes in a double 

 series of plates, and was evidently continued far down the central 

 grooves of the pseudambulacral fields." He also discovered in 

 P. stelliforynis^ a, covering of the central summit aperture "essentially 

 the same as in P. Norwoodii," and he described it as consisting of 

 "five small plates, arranged like a five pointed star, with the points 

 touching each of the u})per ends of the interradial plates, thus 

 completely covering the summit aperture." 



Figures of tlie summit plates of Granatocrinus Norwoodi and 

 Orophocrinus (Codonites) stelUformis were subsequently published by 

 Meek and Worthen, * confirming the observations of Shumard and 



*Archiv f. Naturgesch., 1851. Jahrg. XVII, p. 378. 



1 Palaeontology, in Swallow's Geol. Surv. Mo. 1855, p. 186. 



- Bost. Journ. Nat. Hi.st. 1863, Vol. VII, No. 4, p. 484. 



* ILid. p. 487. 



* lllinoi.s Geol. Rep., Vol. V. PI. IV, figs. 2a, 5. 



