EEPOKT ON THE CRINOIDEA. 39 



plate. Hexacrhms, Dichocrinus, and their allies present a similar condition. In a few 

 genera of the Rliodocrinidse, which have been grouped together into a section Ehodocrinites 

 by Wachsmuth and Springer, the radials are not contiguous laterally ; but between every 

 two there is an interradial plate which rests on a basal below. 



This character, which occurs in no stalked Neocrinoid, either recent or fossil, reappears 

 in the singular Comatulid Thaumatocrinus^ (PI. LVI. figs. 1-4). But with this excep- 

 tion all the primary radials of every adult Neocrinoid, recent or fossil, stalked or free, 

 form a complete ring. 



Calyx-interradials are very usually present in the Palseocrinoids, helping, together with 

 the hio-her orders of radials, to increase the size of the cup, and strengthen its walls. 



According to Wachsmuth and Springer^ "The first interradial is always larger than 

 any of the rest, and is situated between the upper sloping margins of the adjoining first 

 radials, except in some species of the Rhodocrinidse, in which it rests directly upon the 

 basals, separating the ring completely. There are generally two plates in the second 

 series, and two or three in each succeeding one." 



In the Mesozoic genus Guettardicrinus, and in some species of Apiocrinus [Ajnocrinns 

 martini, Apiocrimts roissyanus), there are calyx-interradials essentially similar to those 

 of the Palseocrinoids. Each series commences with a single plate resting upon the upper 

 angles of two first radials which are truncated for its reception (see fig. 9, on p. 183). 

 It is followed by several others, more or less irregularly arranged ; and these, together 

 with the two outer radials, and sometimes also the two lower brachials, form the im- 

 movable wall of a large cup just as in the Palseocrinoids. No recent Crinoid presents 

 this condition, at any rate in the adult state ; though it occurs in many Ophiurids, as 

 pointed out elsewhere.^ But in all the Pentacrinidse, recent and fossil,* the interradials, 

 if present, are not calyx-plates at all, but merely small and more or less irregular 

 plates developed in the perisome which unites the rays and their subdivisions (PI. XIII. 

 fig. 1 ; PI. XXXI. fig. 2 ; PL XXXIV. fig. 1 ; PI. XXXV. fig. 2 ; PI. XXXVII. fig. 1; 

 PI. L. fig. 1). It would seem, however, that regular calyx-interradials may appear in 

 the early larval stages of Antedon rosacea. At any rate this is the way in which I 

 should interpret the following statement by Sir Wyville Thomson.' "In one or two 

 cases, however, I have observed about the time of the first appearance of the anal plate, 

 a series of five minute rounded plates developed interradially between the lower edges of 

 the oral plates and the upper edges of the basals." These plates therefore, separate 

 the radials from one another all round the calyx. Their ultimate fate is uncertain. Sir 



1 On a new Crinoid from tlie Southern Sea, Phil. Trans., 1883, pp. 919-926, pi. 71. 

 ^ Revision, part ii. p. 12. 



3 On the Apical System of Ophiurids, Quart. Journ. Micr. Sci., vol. xxiv., X.S., January 1884, p. 12. 



4 See, for example, Quenstedt's Encriniden, Tab. 101, figs. 23, 39a; Austin's Crinoidea, pi. xiii. fig. Ic ; and 

 Buckland's Geology (Bridgewater Treatise), vol. ii., pi. liii- tig- 2. 



' Phil. Trans., 1865, p. 540. 



