DESCRIPTIONS OF THE SPECIES. 153 



apertures were covered by a Bingle dome-shaped integument composed of very small 

 plates apparently like that previously described in P. Godoni. Additional observations 

 on the structure and position of the pinnules wen- made in 1802 by Prof. James Hall, 

 who appears to have had excellent material for this purpose in Pentremites Whitei ', 

 and also by Dr. C. A. White 2 , who discovered them in P. elongatus, Shumard. lie 

 describes the pinnules as apparently connected directly with the outer side plates, 

 and adds, " They are directed obliquely toward the centre of the pseudambulacral 

 field, for a short distance, and are then bent directly upward, and lie side by side, 

 nearly tilling the field. . . . They appear to be composed of a double series of short, 

 angular plates, with parallel sutures, so arranged that the sutures between the plates 

 of one series are opposite the middle of the plates of the other series." 



The most important researches on the Blastoids made in Great Britain for many 

 years were those published in 1865 by the late Mr. John llofe 3 , but they will more 

 properly be noticed in the history of Granatocrinus. His remarks have reference to 

 the hydrospires and lancet-plate which he was the first to display by the section 

 method in P. Godoni, and indeed in any Blastoid. 



In 1869 the late Mr. Billings described the presence of a small pore at the proximal 

 end of each (partially covered) ambulacral groove in Pentremites conoideics 4 , and he 

 regarded them as ovarian in function. He further supported Lyon's description of the 

 so-called " supplemental basals," and also Bofe's view of the respiratory character of 

 the hydrospires as he proposed to call them. We are likewise indebted to this careful 

 observer for a minute study of the ambulacra oi Pentremites pyriformis, which with 

 the earlier and corresponding observations made by Troost on P. Godoni may be said 

 to have pretty definitely fixed the structure of this part of the Pentremite economy. 



Messrs. Meek and AVorthen 5 have shown that in Pentremites Burlingtonensis the 

 spiracles, usually single in this genus, are double, and appear " at the surface as four 

 pairs of elongate oval pores." They thus depart considerably from the usual structure 

 of the spiracles in Pentremites and approach those of Mesoblastus. The hydrospires 

 of Pentremites have formed the subject of close study by Messrs. Wachsmuth and 

 Springer 6 , who have described these organs in detail, and, like Billings, believe them 

 to have served the same functions as the pectinated rhombs of the Cystidea. They 

 have further detected an important addition to the lancet-plate, in the form of an under 

 lancet-plate, which they describe as perforated by a tubular passage 7 , and they likewise 

 discovered the ambulacral opening between the deltoids (PI. I. figs. 6, 7). 



1 Fifteenth Ann. Rep. New York State Cab. Nat, Hist. 1802, p. 150. 



a Boston Journ. Nat. Hist. 1802, vol. vii. no. 3, p. 488. 3 Geol. Mag. 1805, vol. ii. p. 249. 



4 American Journ. Sci. 1809, vol. xlvii. p. 353, ibid. 1S09, vol. xlviii. p. SI, f. 15. M'Coy had previously 

 suggested the presence of " ocular pores " in this position when describing other species which now fall into 

 Granatocrinus (Brit. Pal. Foss. 1851, fas. 1, p. 123). 



5 Report Geol. Survey Illinois, ls73, vol. v. p. 4r,i'. 



6 Revision of the Palaeocrinoidea, Part I. 1879, p. 7. : Ibid. t. xvii. f. 5. 



X 



