128 



ANNULOIDA. 



calyx. Upon the last of these developed column-joints rest 

 the basal plates of the calyx proper, and upon the margins ot 

 the calyx is inserted a circle of bifid and 

 pinnate arms. Although the calyx itself 

 is of large size, the internal cavity or vis- 

 ceral chamber, in which the soft parts of 

 the animal were contained, is comparatively 

 very small. The best- known species of 

 Apiocrinus are the A. rotundus of the 

 Great Oolite and the A. Roissyanus (fig. 

 81) of the Coral Rag. 



In the Cretaceous period the Apiocrinida 

 are represented by the genus Bourgudi- 

 crinus, and the living genus Rhizocrinus 

 (fig. 72) would appear also to be a repre- 

 sentative of this family. The free Crinoids 

 are pretty well represented also in the 

 Cretaceous period, the White Chalk having 

 yielded the discs of Comatula, along with 

 the singular " Tortoise-encrinite " (Marsu- 

 pites), which would seem to connect the 

 stalked Crinoids with the free forms. 



In the Tertiary period the Crinoids are 

 reduced, so far as is certainly known, to 

 the single genus Pentacrinus, which is re- 

 presented at the present day by the living 

 Pentacrinus Caput-Medusa of the Antilles. 

 The best -known species is the P. sub- 

 basaltiformis of the Eocene. 



A good classification of the Crinoids is 

 still a desideratum. Commonly they have 

 been divided into two groups termed re- 

 spectively Articulata and Tessdata, accord- 

 ing as the radial plates of the calyx are 

 freely articulated to one another, or are 

 immovably joined together without articu- 

 lation. This arrangement is a far from 

 satisfactory one ; but if accepted, we should 

 have all the Mesozoic and Kainozoic 

 Crinoids (except Marsupites] in the divi- 

 sion of the Crinoidea articulata. On the 

 other hand, the division of the Crinoidea 

 tessdata would correspond with the Palaeo- 

 Apiocrinu* Crinoids, comprising all the species known 

 ' from the p al*ozoic formations. 



