34 THE VOYAGE OF H.M.S. CHALLENGER. 
Pentacrinus this expansion takes place near the top of, but entirely within the basal ring, 
the radials 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 (Pl. VIIb. 
festa <4, 53° PL Villa. figs 67s Pie Oxy. ffigs, -7=9 > Pl Vee es 
Pl. LXII.). Within this central funnel of the radials, and closely enveloping the 
plexiform gland, is a kind of plug formed by numerous irregular limestone bars which are 
developed from the inner faces of the radials. In Bathycrinus it is practically little 
more than calcified connective tissue (Pl. VIIb. figs. 1, 4, 5, c), but it becomes very solid 
in Rhizocrinus, and has been wrongly described as a basal rosette (Pl. VIla. fig. 7 ; 
Pl. X. figs. 1, 4,br.). It isalso well developed in Pentacrinus (Pl. XXIV. figs. 8, 9, ”p.), 
and is lodged in a small depression upon the upper surface of the basal ring, which is 
formed by the truncation of the inner ends of its component pieces (Pl. XX. figs. 2-6, 9). 
Among the Apiocrinide 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 Comatule, Atelecrinus and 
Thaumatocrinus; 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 Millericrinus 
nulleri. 
A. Tue Basats. 
The basals of the Neocrinoidea vary considerably in the extent to which they are 
developed. In all the Paleocrinoids the radials are separated from the top stem-joint by 
one complete ring of plates, to which a second is often added. But in the Neocrinoids 
no basals may be visible at all upon the exterior of the calyx, as in most Comatule ; or 
there may be a single complete ring of high plates as in Rhizocrinus (Pl. IX. figs. 1-3 ; 
Pl. X. figs. 2,3; Pl 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 Comatulz 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 has convinced me that in 
two cases’ at any rate the supposed absence of basals in fossil Pentacrinidee is merely 
the result of defective observations ; but this may not be invariably true. No basals are 
visible externally in the Jurassic Isocrinus pendulus, Meyer,’ nor in the Pentacrinus 
pentagonalis personatus from the Brown Jura, which is figured by Quenstedt* without 
1 These are (1) Pentacrinus fisheri, Forbes, from the Kimmeridge Clay of Weymouth ; and (2) a fine specimen 
from the Chalk, which is figured in Dixon’s Geology of Sussex (1878 edition, pl. xix. 22). 
* Tsocrinus und Chelocrinus, Museum Senckenbergianum, Frankfurt, 1837, Taf. xvi. figs. 1, 2. 
3 Encriniden, Tab. 98, fig. 137. 
