EEPOET ON THE CRINOIDEA. 3 



plates whicli make up each of these rings (PI. XII. figs. 1, 2, 11-16, 22-25 ; PL XVIII. 

 figs. 4-7 ; PI. XX. figs. 1-3, 6-9 ; PI. XXI. figs. 6, 7 ; PL XXIV. fig. 6, lb ; fig. 7, 1, lb, L ; 

 PL LVIII. fig. 1, /, lb; fig. 2, L). The radials of Bathycrinus are likewise united in this 

 way (PL Vllb. fig. 4, 1) ; but the sutures between the basals seem to be of a still closer 

 nature. They are visilile externally in young individuals but disappear in the adult, so 

 that the "base " seems externally to consist of but one single piece^ (PL Vila. figs. 12-14). 

 Sections through a decalcified specimen show, however, that it is really composed of five 

 parts like the base of Pentacrimis (PL Vllb. fig. 2, B). These five parts are, nevertheless, 

 very closely anchylosed. No parallel fibres of connective tissue pass between them, such 

 as unite the five first radials together (PL Vllb. fig. 4, I). Pnit the organic basis of the 

 skeleton is much less close, if not absent altogether, along five lines which radiate out- 

 wards from the chambered organ and indicate the position of the sutures. They stain 

 less deeply with hsematoxylin than the surrounding tissue, but do not reach the exterior 

 of the section (PL Vllb. fig. 2). Neither are they visible in sections through the top 

 and bottom of the basal piece ; and but for the knowledge obtained in this way, the 

 basal piece would probably have been described as an uppermost stem-joint, as has 

 actually happened in the case of Rhizocrinus. 



In some individuals of Rhizocrinus lofotensis (PL IX. figs. 1, 2) there are no 

 indications of suture, even in the adult ; though in others the sutures are visible 

 externally (PL X. fig. 2). In the former case the nuclear connective tissue network is 

 continuous through the whole section, which exhibits no unstained radiating lines, as do 

 similar sections of Bathycrinus. There is a marked diflerence between the two genera, 

 however, as regards the lateral union of the radials. Those of Bathycrimis are united by 

 synosteal fibres (PL Vllb. fig. 4, /), just as in Pentacrinus (PL XXIV. figs. 7-9 ; 

 PL LVIII. fig. 2, L) and Comatiila. But those of Rhizocrinus are much more closely 

 connected, just in fact in the same way as the basals of Bathycrinus are. No ligamentous 

 fibres are visible in horizontal section, but only five (or six) radiating lines where the' 

 nuclear network is incomplete (PL Villa, fig. 6). The radials in this type, and the 

 basals in Bathycrimis, seem to be united by a limestone deposit which has difl'erent 

 characters from that usually found in Echiuoderms ; and I think it will be advantageous 

 to denote this by the term " anchylosis," reserving " synostosis " for cases in which 

 ligamentous fibres are concerned in tlie union of the joints ; though both of them, like 

 the " syzygy," would be considered as sutures. 



Smooth sutural unions like those between the basals and radials, though somewhat 

 less close in their character, occur at intervals in the stem of the Pentacriuidfe, every 

 nodal or cirrus-bearing joint being united in this way to the joint below it (PL XIX. 

 figs. 3, 4; PL XXIL figs. 16, 20, 21; PL XXVL figs. 12-16; PL XXXL fig. 3; 



> This must be cirefully distiuguished from the "article basal" of the ApiocrinUiu, which is the uppermost 

 stem-joint that supports the basals. 



