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



expanded ends, while the articular ridges in the long axes of the terminal faces cross one 

 anotlier at various angles. 



Tliis mode of articulation is common to all the Bourgueticrinidie, though the stem- 

 joints are not always so long as in Rhizocrimis and Bathycrinus. It occurs also in the 

 curious senus TJiioHiericrimts and in the stem of the larval Comatida. 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 amono; the diiferent members of the 

 Bourgueticrinidge in the characters of the terminal faces of the stem-joints. In the 

 Jurassic genus TliiolUericrinus, 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 TliiolUericrinus, Mesocrinus, and Bourgueticrinus ellipticus the ligament-fossse 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 (PI. 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 Rhizocrimis 

 (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- 

 crinus ellipticus or Mesocrinus. The two ligament-fosspe 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 Eiigen under the name of Apiocrinus constrictxis. 



'^ Eevision of the Palreocrinoidea, part ii., Proc. Acad. Nat. Sd. Philad., 1881, p. 69 (243). 



2 In a stem-joint from the Msestricht Chalk, which is figured by Quenstedt as Apiocrinus {Bowguetierinus) 

 ellipticus (Encriniden, Tub. 104, fig. 70), there is no articular ridge at all, but merely an oval articular siirfoce 

 around the opening of the central canal. Unless this be the result of an accMental removal of the ends of the 

 articular ridge, it is a somewhat striking peculiarity which tends to approach the condition of the middle stem- 

 joints in Bathycrinus, and has a still closer resemblance to a form of articular siu'face which is especially characteristic 

 of the cirrus-joints {ante, pp. 7, 8). 



3 Encriniden, Tab. 104, tigs. 64-66. 



