250 PALEONTOLOGY 



grooves, converging to an exposed mouth. An inter- 

 radial anus would be the only disturbance of symmetry. 



Isolated plates of Marsupites can easily be recognized 

 by their curious ornament of ridges arranged in sets at 

 right angles to the sutures. An almost identical pattern 

 is found, however, in the Silurian Crotalocrinus. 



There is much difference of opinion as to the classifi- 

 cation of crinoids. Dr. Bather, of the British Museum, 

 regards the distinction between monocyclic and dicyclic 

 forms as fundamental, whilst most other authors treat it 

 as of merely family value. However sound this distinc- 

 tion may be on morphological grounds, it has the 

 practical difficulty that many dicyclic crinoids are 

 " pseudo-monocyclic," so that the ordinary student does 

 not recognize their dicyclic character. 



In both monocyclic and dicyclic forms certain definite 

 grades of structural complexity are found, which are 

 taken as a basis for division into orders : (i) Inadunata, 

 in which there is no incorporation of fixed brachials in 

 the dorsal cup ; (2) Camerata, with the tegmen in the 

 form of a rigid vault covering the mouth and food- 

 grooves (Palaeozoic only); (3) Flexibilia or Articulata, 

 in which the tegmen is composed of small plates loosely 

 articulated, and the mouth and food-grooves exposed 

 (almost exclusively Mesozoic and later). But authorities 

 differ considerably as to the limits of these orders. 



Among the simplest Inadunata (Larviformia of Wachs- 

 muth and Springer) is the dicyclic Cupressocrinus, already 

 described. Pisocrinus (Fig. 71, a, Sil.) is a monocyclic of 

 equally simple structure, but much smaller sized cup, 

 though the very slender arms attain as great a length as 

 those of Cupressocrimis ; the cup-plates have their syrn- 

 metry disturbed by the subdivision of the right posterior 

 radial obliquely into two plates (r.p. and'R') a common 



