THE STEM WD 0AL1 \. 37 



this type in the National Collection, we find that each of them has a deltoid ap- 

 pearing externally in the anal interradius, though the suture-lines between it and the 

 radials below are somewhat obscure. Together with Eambach's example, therefore, 



six individuals have been seen which present tins peculiarity, viz. four deltoids 

 limited to the summit, like those of Pentremitidea Wachsmuthi (PI. V. figs. 6, 7), 

 and one appearing externally on the anal side like those of /'. leda I (PI. V. fig. 12). 

 Should this prove to be a universal character, it will be necessary, as Mr. Wachsmuth 

 remarks, to separate Troostocrinus Beinwardti generically from '/'. lineatw, T. Gros- 

 venori, &c, a point to which we shall return in the systematic portion of the 

 catalogue. 



F. Ox the Homologies of the Deltoid Plates. 

 The deltoid plates of a Blastoid are now generally recognized as representing in a 

 more or less collective manner the interradial system of a Crinoid. In the genus 

 Cyathocrinus this is constituted by five relatively large plates which rest on the 

 upper edges of the radials and form the boundary of the peristome; while the 

 ambulacra pass inwards over the grooves formed by their apposed and truncated 

 edges. These grooves are converted into tunnels by the alternating covering-plates, 

 which extend outwards in a radial direction from the group of summit-plates, just as 

 is the case with the ambulacra of the Blastoids (PI. HI. figs. 2, 3 ; PL VII. figs, 3, 4, 

 10-13 ; PI. XV. figs. 11, 12). In C/fat/tocrinus, as in most Blastoids, there docs not 

 appear to be any great amount of regularity in the arrangement of the summit- 

 plates. But in Eheacrinus it is not uncommon for each of the four normal interradii 

 to have a single summit-plate corresponding to it, and two or more in the anal inter- 

 radius, as shown in PL XVIII. fig. 10. Roemer 1 figures an extremely regular 

 instance of this ; while in Hall's diagram of E. eletjans' 2 only one summit-plate is re- 

 presented on the anal side, so that the total number does not exceed five, a condition 

 which is constant in Stephanocrinus (PL XIX. fig. 9). The summit-plates, therefore, 

 are typically separated from the circle of united radials by one single plate, the 

 deltoid, in each interradius (Fig. IV. A.). This condition recurs in the simpler forms of 

 Platycrinus, which have small interradials with the proximal dome-plates resting 

 directly upon them (Fig. IV. B) ; and if the views be well founded which have long 

 been advocated by one of us respecting the homology of these proximal dome-plates 

 in Platycrinus with the orals of Neocrinoids and the mouth-shields of Ophiurids, 

 there are many Ophiurids which are morphologically very similar to Platycrinus 

 and the Blastoids. Thus, for example, in Ophiomusium lunare the large radial 

 primaries on the dorsal surface of the disc are only separated from the mouth-shields 

 on the ventral aspect by two interradials. There are three of these plates in 



1 Archiv f. Xaturgesch. 1851, Jahrg. xvii. Ud. i. p. 378, taf. v. figs. 16, lc. 



2 Fifteenth Ann. Eep. New York Htato Cab. Nat. Hist. Albany, lb(J2, p. 153. 



