34 THE VOYAGE OF H.M.S. CHALLENGER. 



Pentacrinus this expansion takes place near the top of, but entirely within the basal ring, 

 the raclials having no share in the protection of the chambered organ, though they 

 surround the lowest portion of the plexiform gland which rises out of it (PI. Vllb. 

 figs. 1, 4, 5; PI. Villa, figs G, 7; PL XXIV. figs. 7-9; PL LVIII. figs. 1-3; 

 PL LXIL). Within this central funnel of the radials, and closely enveloping the 

 2)lcxiform gland, is a kind of plug formed by numerous irregular limestone bars which are 

 dcve]o])ed from the inner faces of the radials. In Bathycrinus it is practically little 

 more than calcified connective tissue (PL Vllb. figs. 1, 4, 5, c), Init it becomes very solid 

 in Rhizocnnus, and has been wrongly described as a basal rosette (PL Vila. fig. 7 ; 

 PL X. figs. 1, 4, hr.). It is also well developed in Pentacrinus {2\. XXIV. figs. 8, 9, rp.), 

 and is lodged in a small depression upon the upjjcr surfixce of the basal ring, which is 

 formed by the truncation of the inner ends of its component pieces (PL XX. figs. 2-G, 9). 

 Among the Apiocrinida^ the cavity which lodged the chambered organ is bounded in 

 varying proportions by the basals and radials. In some species, such as Millericrinus 

 milleri, this structure must have lain altogether upon the ventral aspect of the basals. 

 These form a complete ring, just as in the aberrant Comatula;, Atelecrinus and 

 lliaumatocrimis ; but in both these types the basal ring is on the ventral side of the 

 chambered organ, which is precisely the opposite condition to that of Millericrimts 

 milleri 



A. The Basals. 



The basals of the Neocrinoidea vary considerably in the extent to ^\'llich they are 

 developed. In all tlic Pala3ocrinoids the radials are separated from the top stem-joint by 

 one complete ring of plates, to which a second is often added. But in tlie Neocrinoids 

 no basals may be visible at all upon the exterior of the calyx, as in most Comatula? ; or 

 there may be a single complete ring of high plates as in Rldzocrinus (PL IX. figs. 1-3 ; 

 PL X. figs. 2, 3 ; Pi. LIII. figs. 7, 8) ; or there may be two rows of plates of variable 

 size as in Encrinus, Extracrinus, and Marsupites. 



The absence of external basals in most recent Comatula3 is due to their having 

 undergone metamorphosis into the well-known rosette, which is concealed between the 

 centro-dorsal and the radials. But the occurrence of this condition in a stalked Crinoid 

 would appear somewhat improbable. Personal examination luis convinced me that in 

 two cases * at any rate the supposed absence of basals in fossil Pentacrinidse is merely 

 the result of defective observations ; but this may not be invariably true. No basals are 

 visible externally in the Jurassic Isocnnus pendxdus, Meyer,^ nor in the Pentacnnus 

 pentagonalis personatus from the Brown Jura, which is figured by Quenstedt' without 



' These are (1) Pcnfacji'jiws /i.s7i.«ri, Forbes, from the Kimmeridge Chiy of Weymouth ; aiul (2) a fine speeinieu 

 from the Chalk, which is figured in Dixon's Geology of Sussex (1S78 edition, pi. xix. 22). 



' Isocrinua und Chelot-rinus, Museum Senckenbergiaiiuni, Frankfurt, 1637, Taf. xvi. figs. 1, 2. 

 " Eueriiiiden, Tab. 98, iig. 137. 



