24 THE VOYAGE OF H.M.S. CHALLENGER. 
expanded ends, while the articular ridges in the long axes of the terminal faces cross one 
another at various angles. 
This mode of articulation is common to all the Bourgueticrinide, though the stem- 
joints are not always so long as in Rhizocrinus and Bathycrinus. It oceurs also in the 
curious genus Thiolliericrinus and in the stem of the larval Comatula. Both Thiollieri- 
crinus and Bourgueticrinus occur in the Jurassic rocks ; while the same kind of column as 
occurs in these genera existed also in the Carboniferous Platycrinus, and according to 
Messrs. Wachsmuth and Springer’ “forms one of the most characteristic features 
of the genus.” 
There is a considerable amount of variation among the different members of the 
Bourgueticrinidee in the characters of the terminal faces of the stem-joints. In the 
Jurassic genus Thiolliericrinus, in Bourgueticrinus (Jurassic and Cretaceous), and in the 
Cretaceous Mesocrinus the articular ridge is narrow and linear, expanding somewhat 
around the opening of the central canal to form the real articular surface. ” 
In all these genera a median groove extends along each half of the ridge, from the 
central opening towards the margin of the joint face ; and short shallow branches proceed 
from it on either side so as to cut out the upper portion of the ridge into a double row of 
small teeth. 
In Thiolliericrinus, Mesocrinus, and Bourgueticrinus ellipticus the ligament-fosse at 
the sides of the articular ridge are either uniformly shallow throughout their whole extent, 
or they are deepest in the immediate neighbourhood of the central canal. But they are 
completely separated from one another by the articular ridge, which is continuous from 
end to end of the elliptical surface. Very much the same is the case in the upper and 
middle stem-joints of Bathycrinus (Pl. VIla. figs. 8, 9), except that the articular ridge 
is relatively larger and is destitute of teeth. But in the lowest stem-joints of this genus 
(Pl. VII. figs. 12, 13; Pl. Vila. fig. 11), and in all parts of the stem of Rhizocrinus 
(Pl. X. figs, 11-14), the articular surface is incomplete, and instead of surrounding the 
central canal, is actually divided by it into two trihedral portions, the upper edges of 
which are toothed just like the corresponding parts of the complete ridge in Bourgueti- 
erinus ellipticus or Mesocrinus. The two ligament-fossee communicate with one another 
around the opening of the central canal, which thus appears to lie at the bottom of a 
deep depression. Quenstedt ° figures some stem-joints of this kind from the white chalk 
of Riigen under the name of Apiocrinus constrictus. 
1 Revision of the Paleocrinoidea, part ii., Proc. Acad. Nat. Sci. Philad., 1881, p. 69 (243). 
* In a stem-joint from the Mestricht Chalk, which is figured by Quenstedt as Apiocrinus (Bourgueticrinus) 
ellipticus (Encriniden, Tab. 104, fig. 70), there is no articular ridge at all, but merely an oval articular surface 
around the opening of the central canal. Unless this be the result of an accidental removal of the ends of the 
articular ridge, it is a somewhat striking peculiarity which tends te approach the condition of the middle stem- 
joints in Bathycrinus, and has a still closer resemblance to a form of articular surface which is especially characteristic 
of the cirrus-joints (ante, pp. 7, 8). 
3 Encriniden, Tab. 104, figs. 64-66. 
