BASAL PLATES IN CRINOIDEA CAMERATA 537 



the formation of the compound, left-posterior radial in Anomalo- 

 crinus, 1 yet Springer 2 has shown it to be an abnormality, due per- 

 haps to plate fracture? There are, it is true, certain conditions 

 surrounding apparent cases of plate division which lead us to 

 believe that division and not interpolation has occurred. If, for 

 example, two plates of the same cycle occupy the approximate 

 area of one plate in that cycle, if they mutually fulfil the require- 

 ments of but one plate in that cycle, and if in the obliteration of 

 the intervening suture a plate would be formed indistinguishable 

 from the other four undivided plates in that cycle or from the 

 morphologically undivided equivalent of that plate in a closely 

 related ancestral genus, division would seem the only logical con- 

 clusion. But this conclusion is by no means proved. In the 

 light of phylogenetic and ontogenetic development as ascertained 

 from fossil crinoids division is very uncertain, for we cannot see 

 the process taking place. Furthermore, if division had occurred 

 anchylosis of the parts to the adjacent plates might take place and 

 either complete absorption with compensating enlargement or 

 migration would have to be called upon to explain the appearance 

 of the new suture. Thus, no matter how carefully we attempt to 

 ascertain the fact that division has occurred, the factor of inter- 

 polation will usually appear as an alternative. Only through 

 careful observation, in modern larval development, of plates not 

 destined to obliteration in the adult stage can this process be 

 satisfactorily determined. 



d) Plate migration. — -Any shifting which brings a plate, as a 

 unit, into a new relation of contact and position with plates of the 

 adjacent cycles, or of adjacent plates in the same series, may be 

 termed a migration. For simplification in the discussion, the dif- 

 ferent types of, migration may be broadly separated into two 

 divisions: simple migrations, or those unattended by movements 

 of the sarcode; and complex migrations, or those dependent upon 

 movements of the sarcode. Of the simpler forms, three types 

 occur: portional migration, cell-group migration, and simple plate 

 migration. 



1 Ref. 37, Part III, p. 221; ref. 40, p. 152. 



2 Ref. 32, p. 213. 3 The italics are the writer's. 



