66 CATALOGUE OF THE BLASTOIDEA. 



CHAPTER IV. 



THE SUMMIT-PLATES. 



The first intimation that the summit of an ordinary Blastoid was not naturally as 

 free as it appears to be in by far the larger number of specimens which have been 

 obtained, was afforded by the discovery of Owen and Shumard 1 that the mouth 

 and ovarial apertures in a specimen of Pentremites Godoni were " in the perfect state 

 completely closed by a conical covering of small plates." 



In the same year (1850) Roemer 2 discovered that the central opening of Stepha- 

 nocrmus is closed by a pyramid of five pentagonal plates ; though as he referred 

 this genus to the Cystidea, his discovery, which was soon confirmed by Hall 3 , 

 attracted but little attention from students of the Blastoidea. In the next year, 

 however, he published his celebrated description of the summit of Plceacrinus 

 Verneuili, which he described as closed by six plates enclosing a hexagonal one in 

 the centre 4 . He had seen this covering, though not in all cases the sutures 

 between the plates, in some thirty individuals, and his general description of it has 

 been confirmed by all subsequent writers on the genus. Among these was Shumard 5 , 

 who in 1855 concluded his diagnosis of Pentremites Sayi in the following terms: — 

 " The central opening is closed by minute, usually pentagonal and hexagonal plates 

 arranged in a manner somewhat similar to those of Pentremites {Flceacrinus) Ver- 

 neuili (Roemer) ;" and he added in a note — " The same structure occurs in Pentre- 

 mites Norwoodi and P. melo, Owen and Shumard, of which I have fully satisfied 

 myself from an attentive examination of many specimens." 



Hambach 6 , however, has made the somewhat emphatic general statement that the 

 central opening "was never closed by additional plates, as intimated by some authors 

 (Billings and Shumard) ; although specimens are frequently found (and I have such 

 in my collection) where it appears as if the summit were closed by additional 



1 .Tourn. Acad. Nat. Sci. Philad. 1850, vol. ii. pt. 1, p. 65; and Trans. St. Louis Acad. Sci. 1858, vol. i. 

 no. 2, p. 243. Wo have preferred to quote the later description of this fossil which was given by Shumard, 

 rather than the original statement by himself and Owen, in which no reference to the mouth is mado. 



- Archiv f. Naturgesch. 1850, Jahrg. xvi. Bd. i. p. 3G9, Taf. v. fig. 3. 



3 Palaeontology of New York, 1852, vol. ii. pp. 212, 213. 



4 Archiv f. Naturgesch. 1851, Jahrg. xvii. p. 378, Taf. v. fig. 1 e. 



5 " Palaeontology," in Swallow's First and Second Annual Report Geol. Survey Missouri, 1855, p. 186. 



6 Trans. St. Louis Acad. Sci. 1880, vol. iv. no. 1, p. 150. 



