MORPHOLOGY 53 



well shown in the leading Taxocrinoid genera from earliest to latest; as in 

 Protaxocrinus (PI. XLV, figs. 15, 16), and Taxocrinus (Pis. LIII, fig. 3; 

 LVII, fig. 3a; LIX, fig. 10). In Plate LVII, figs. 8a, b, the perfectly rounded 

 distal face of the strong interbrachials may be seen, and in 6b the slight groove 

 is shown for attachment of the perisome, remnants of which are scattered over 

 the disk in figure 6a from the same specimen. This was also the case with the 

 earlier Forbesiocrinus (PL XXIII, fig. ia), and with Temnocrinus (PI. XVI, 

 fig. 5). In Sagenocrinus, and the later Forbesiocrinus, the solid interbrachials 

 pass high up between the rays and seem to merge insensibly into the tegmen. 

 In Pycnosaccus and Nipterocrinus there is nothing but perisome between the 

 rays clear down to the radials. In Lithocrinus strong interbrachial plates in 

 the lower part of the interradius are isolated by perisome surrounding them. 

 These conditions may be compared with that of the Recent Ptilocrinus, in 

 which the rays are separated, from the radials up, by areas of perisome in 

 which a number of good-sized plates appear, rather irregularly distributed 

 (PL LXXV, figs. \a, b). With these well-filled interbrachial spaces, the cen- 

 trally located orals with the posterior one modified by lateral anal structures, 

 and the strong ambulacra — this genus bears a stronger resemblance to a 

 Paleozoic crinoid than any adult Recent form yet discovered. 



The function of the interbrachial plates was to fill up spaces between the 

 radial elements as required by the enlargement of the calyx in the growing 

 animal. In rare cases they are limited to a single large plate in each inter- 

 radius, with the rays arching above it, as in Anisocrinus (PL X, fig. 3a) ; and 

 there are all variations from that type to the numerously plated areas of 

 Sagenocrinus and Forbesiocrinus. In the Flexibilia there is no plate of the 

 interradial series, except in the posterior interradius, entering the radial circlet 

 and passing down to contact with the basals as in the Camerate families 

 Rhodocrinidae and Reteocrinidae, and in the living genera Proinachocrinus 

 and Thaumatocrinus. That structure is a phase in the morphology which ap- 

 pears to be unrepresented in this group, save in an isolated species among the 

 later Taxocrini, and occasionally in individuals of one or two others. 



It was for a long time supposed that the interradials were a structure 

 sui generis, belonging almost exclusively to the Paleozoic crinoids. Some 

 traces of their development were found in the course of the researches upon 

 the larva of Antedon. Sir Wyville Thomson 1 describes their occurrence as 

 follows : 



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 interradial plates 

 sometimes remain permanent in the mature Antedon rosacnts, and they appear to be con- 



1 Philosophical Transactions, Royal Society, London, 1865, p. 540. 



