CRINOIDEA. 271 



the mouth in concealed channels or tunnels, in having mostly 

 a rounded column, and in the fact that the articular facets 

 of the column-joints are usually marked by numerous simple 

 radiating striae. As a rule, also, the Palseocrinoids have a 

 calyx, in which the dorsal surface has a preponderating 

 development as compared with the ventral surface. 



Coming to Mesozoic times, Crinoids are still abundant, 

 though certainly reduced in numbers as compared with their 

 development in the Palaeozoic period. Taken as a whole, 

 the Mesozoic Crinoids are characterised by having the brachial 

 grooves continued over the ventral surface of the disc, and 

 by having the ventral portion of the calyx more extensively 

 developed than the dorsal ; whilst the articular surfaces of 

 the column-joints are usually morticed to one another by 

 means of crenated ridges which have a flower-like arrange- 

 ment. Neither of these latter characters, however, is con- 

 stant. In the Mesozoic rocks, also, appear for the first time 

 remains of the free Crinoids allied to the living Comatulce. 



In the Tertiary period the Crinoids are reduced to a very 

 small number of forms, the most important of which is the 

 genus Pentacrinus, which is represented at the present day 

 by the living Pentacrinus caput-Medusce of the Antilles and 

 various other species. The best-known species is the P. sub- 

 lasaltiformis of the Eocene. At the present day, Crinoids 

 are moderately abundant, if we look only to the free forms 

 like Comatula, which are largely represented, and attain 

 their maximum in recent seas. On the other hand, the 

 stalked Crinoids are reduced to a comparatively very limited 

 number of generic types represented by a comparatively 

 small number of species. 



A good classification of the Crinoids is still a desideratum. 

 Commonly they have been divided into two groups termed 

 respectively Articulata and Tesselata, according as the radial 

 plates of the calyx are freely articulated to one another, or 

 are immovably joined together without articulation. This 

 arrangement is a far from satisfactory one ; but if accepted, 

 we should have all the Mesozoic and Kainozoic Crinoids 

 (except Marsupites) in the division of the Crinoidea arti- 

 culata. On the other hand, the division of the Crinoidea 



