R. ETHERIDGE, JUX., ON ASTROCRINITES. Ill 



plates, although called " radials," do not support the bases of the pseud- 

 ambulacra as do those plates in the Crinoidea, but, on the contrary, 

 these are situated at the apex of the Pentremite, the smaller extre- 

 mity of each corresponding with the " apex of the ambulacrum of a 

 Sea-urchin or of a Starfish. It also represents the tip of the arm of 

 a Crinoid." Mr. Billing's deduction from this is, that the " forked 

 plates do not belong to the radial but to the perisomatic system." 

 In a similar manner the bases of the pfcudarnbulacra in Astrocrinites 

 are situated at the apex of the fossil, tho tips being received in the 

 " radial " or forked plates, occupying the position of the bases of the 

 arms of a Crinoid *. The three convex arched plates (b, fig. 5) are 

 probably analogous to the interradials in the Blastoidea (or, fig. 27), 

 whilst for the truncated plate (figs. 13 & 14, a, &fig. 20) partly sur- 

 rounding the excentric aperture it is more difficult to find a homo- 

 logue. I would ask, Can it be compared to the fifth interradial 

 plate of Nucleocrinus (which, according to Mr. Billings, " is trun- 

 cated at its apex for the reception of the oro-anal orifice "f)? This 

 oro-anal area of Nucleocrinus, Messrs. Meek and Worthen tell us, is 

 wider and " often more prominent above . . . , and occupied 

 by three large, elongated pieces, the middle one of which, the anal 

 piece, is lanceolate in form, and, with the two interradials, fills all 

 the large anal area down to the base" J. 



I have not succeeded in finding any specimen with the central 

 opening closed by small plates as in Nucleocrinus §, Granatocrinus ||, 

 and some Pentremitce % ; nor are there any apertures to be seen 

 analogous to those occurring at the apices of the deltoid or inter- 

 radial plates of these genera, the ovarian orifices of some authors, 

 the " respiratory spiracles " of Mr. Billings **. Finally, I believe 

 that the peculiar lid or valve (h, fig. 5, g, fig.20) covering the inter- 

 radial aperture in Astrocrinites is analogous to that figured ff by 

 Messrs. Meek and Worthen as covering the so-called anal aperture 

 (the oro-anal of Billings) in Granatocrinus Norwoodi (Owen and 

 Shumard), although in position it corresponds with the anal opening 

 of Codonaster, McCoy, and Codonites, Meek and Worthen. Both these 

 genera have been transferred by Mr. Billings to the Cystoidea ££. 



The structure of the ambulacra appears to be to a great extent 

 on the plan of the Blastoidea. Mr. Billings has described those of 

 Pentremites pyriformis at some leugth. He says, "The median 

 groove . . . sends off branches, right and left alternately, towards 

 the sides of the ambulacrum. These branches do not run directly 

 to the ambulacrum-pores. Each of them terminates at a point 

 between the inner extremities of two of the pores. There is at this 



* Geol. Surv. Canada, Pal. Foss. ii. pt. i. p. 112. 



t Op. cit. p. 115. | Illinois Geol. Kept. ii. p. 275. 



§ Billings, op. cit. p. 117 and p. 114, f. 73. 



j[ Meek & Worthen, I. c. v. t, 9. f . 2 a & b. 



\ Billings, op. cit. p. 103. 



** Ibid. tt L.c.t.9. f. 2 b. 



|| See his views, American Journ. Sc. 1869, xlviii., 1870, xlix. ; Ann. & Mag. 

 Xat. Hist. 1870, v., 1871, vii. ; Mem. Geol. Surv. Canada, Pal. Foss. ii. pt. i. 

 p. 101. 



Q. J. G. S. No. 126. i 



