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



are almost the only living Crinoids with arm-bases anything like so massive as in 

 the fossil species. The Miocene Antedon rhodanica has a very large centro-dorsal ; but 

 the three species from the Norwich Crag and the two from the Italian Tertiaries are all 

 craite small. 



Another character which presents itself in a large number of the Jurassic and 

 Cretaceous species is the retention of the five-rayed perforation on the lower surface of the 

 centro-dorsal, the peculiarities of which have been discussed in Chapter II. 



The geological distribution of the three fossil genera of Comatulse is shown in 

 the foregoing table. 



