376 Mr. F. A. Bather on British Fossil Grinoids. 



Arm-characters in general may, however, be used as a check 

 on other methods of classification ; they enable us to correct 

 possible errors in phylogeny, for instance a pinnulate form 

 cannot be the ancestor of one with simple arms. In this 

 aspect their study proves of great importance. 



D. Modes of Union. 



The different varieties of suture and articulation by which 

 the plates and ossicles of Crinoidea are held together will be 

 found explained in the section on Terminology (p. 314, ante) 

 under the words Syzygy, Close Suture, Loose Suture, and 

 Muscular Articulation ; repetition is therefore unnecessary. 



On the variations of these structures in different genera 

 Messrs. Wachsmuth and Springer have laid some stress (Rev. 

 III. (189) Proc. 1886, p. 113), and have taken the greater 

 differentiation of articulation in their family Poteriocrinidas as 

 " a good distinction " between them and their family Cyatho- 

 crinidaa. These differences are, however, rather of degree than 

 of kind ; loose suture and even articulation may take the 

 place of a close suture whenever there is any need for greater 

 movement ; while on the other hand a sutural union may 

 become syzygial when greater firmness is required. It 

 would certainly not be philosophical to separate a genus 

 because certain of its articulations were better developed than 

 those of the other genera of its family. Undoubtedly, how- 

 ever, some amount of regular evolution is observable in this 

 point ; later forms, as a rule, combine firmness and free motion, 

 while earlier forms are but slightly flexible. 



A difference of union, correlated with a difference in arm- 

 branching already pointed out, is worth notice. In dichoto- 

 mous pinnulate arms the several axillaria are united to the 

 succeeding plates by muscular articulation ; in simply bifur- 

 cating pinnulate arms there is only one axillare that can be so 

 united. Thus the more cumbrous clichotomous arms have 

 their powers of movement equalized with those of the simply 

 bifurcating arms {Decadocrinus, Scytalecrinus) . But in some 

 later genera of the simply bifurcate type the balance is again 

 brought over by the articulation of several of the proximal 

 arm-plates ( Gi'aphiocrinus, Erisocrinus, Eupacliycrinus) , such 

 as never takes place in dichotomous pinnulate arms. 



Peinciples of Classification. 



The chief variations in the structure of the Fistulata con- 

 sidered, it only remains to be seen how we can best frame a 



