io6 



THE CRINOIDEA 



summarised in the annexed diagram (Fig. XIV.) and table, is in itself 

 empirical, applicable only to pentagonal stems or lumens, and even 

 then liable to exceptions (marked * in table) ; but by attending 



DICYCLIC 



MONOCYCLIC 



Fio. XIV. 



Comparison of the Dicyclic and Monocyclic base. B, basal ; Br, brachials marking the five 

 rays ; ci, cirri, only three out of the tive are shown ; co, pentameres of column ; IB, infrabasal ; 

 n, nerves going to cirri from extensions of capsule ; R, radial ; s, sutures between penta- 

 meres of stem. 



(as is here done) chiefly to the relations of the axial cords, we shall 

 have a surer guide for discrimination between monocyclic and 

 dicyclic crinoids in the many doubtful cases that occur. 



The primitive crinoid is attached by the distal end of its stem ; 

 and it is supposed by many, from the evidence of the embryo 

 Antedon (Fig. XV.), that there is developed at that point a special 

 fixing plate, to which they apply the term " dorso-central," which 

 must not be confused with * 'centre- dorsal " (see especially the 

 writings of P. H. Carpenter). Palaeontology does not lead us to 

 regard such a structure as primitive, or to ascribe to it any morpho- 

 logical importance. As a rule, skeletal growth takes place at the 



