270 CRINOIDEA. 



in other cases succeeded directly by a series of two or three 

 rows of plates, which are directly superimposed upon one 

 another, and which form the foundations of the arms (r, r). 

 These are termed the " radials " (the " costse " of Miller), and 

 are termed " primary," " secondary," and " tertiary," accord- 

 ing to their distance from the basals. The last radial plates, 

 or those furthest from the column (sometimes called the 

 " axillary radials "), give origin to the arms. The radial 

 plates are arranged in a series of vertical columns, which ex- 

 tend from the summit of the basals to the bases of the arms. 

 Between the different columns of radial plates, however, 

 there may be intercalated certain other smaller plates, which 

 alternate with one another, and which are termed " inter- 

 radials " (i). Lastly, one of the inter-radial spaces, corre- 

 sponding with the anus, is usually much wider than the 

 others, and is furnished with an additional series of plates, 

 which are called the " anal plates " (a). 



As regards their general distribution in time, the Crinoidea 

 present us with an excellent example of a group which early 

 attained its maximum of development, and which has now 

 dwindled down to some half-dozen surviving genera. With 

 one or two doubtful exceptions, the Crinoids appear, so far 

 as yet known, to have commenced their existence in the 

 Lower Silurian period, and they are represented by numerous 

 and very varied forms in the seas of the Upper Silurian 

 period. In the Devonian rocks, also, Crinoids are plentiful, 

 and many generic types are peculiar to this period. It is in 

 the earlier portion, however, of the Carboniferous period that 

 the Crinoids attain their highest development. The Car- 

 boniferous Limestone is in many places, over wide areas, and 

 for a thickness of many yards, almost entirely made up of 

 the debris of Crinoids ; and in many places it is so charged 

 with the remains of these organisms as to deserve and acquire 

 the name of " crinoidal limestone " or " entrochal marble." 

 It is in the Palaeozoic period, then, that the Crinoids attain 

 their maximum, both numerically and as regards the number 

 of genera and species. Taken as a whole, the Palaeozoic 

 Crinoids are distinguished by the characters already men- 

 tioned namely, by having the brachial grooves conveyed to 



