THE CRINOIDEA 



155 



Fio. LXVIII. 



Saccocomn. 1, S. teneUa, from aboral surface (after Jaekel, x {). 2, S. pectin/ata, from 

 aboral surface, to show coiling of arm-branches (x ). 3,'S. pectinata, cup and proximal 

 brachials from side (after Zittel, x |). 



ORDER 2. Adunata (Bather, 1899). 



Monocyclica with dorsal cup primitively confined to the patina and 

 an occasional single anal ; tegmen solid ; portions of the proximal Br 

 and their Amb tend to be rigidly incorporated in the theca. Arms fork 

 once to thrice, and bear pinnules on each or on every other Br. BB fused 

 to 3, 2, or 1. (Eucladocrinus and Acrocrinidae offer peculiar exceptions 

 to this diagnosis.) 



The genera of this order have been referred, in whole or part, to the 

 Camerata or their equivalents by most recent writers. But most of the 

 Silurian genera here included are admitted as close allies of Monocyclica 

 Inadunata. If so, then they were derived from Inadunata ages after the 

 typical Camerata had appeared. Moreover, though the Devonian and 

 Carboniferous descendants of these Silurian genera became modified after 

 the fashion of the Camerata, there is scarcely one in which the modification 

 is so great as in the simplest Camerate. The order is divisible into two 

 groups A. with RR in contact all round the cup, and base consequently 

 pentagonal in outline ; B. with RR separated by an anal plate in post. 

 IR, and base consequently hexagonal in outline. The relations of 

 these 'groups are not clear. B. may be derived from A. by sudden inter- 

 calation of an anal, or A. and B. may have descended independently from 

 Inadunata ; the absence of B. from Silurian rocks renders the former 

 hypothesis more probable. There is no difficulty in imagining the descent 

 of early genera of -4., such as Cordylocrinus and Goccocrinus, from Hybo- 

 crinidae in which the cup had become as symmetrical as in, say, Stephano- 

 crinus while the arms had passed through regular dichotomy to a stage 

 with ramuli, as in JEctenocrinus. The actual intermediate form is not yet 

 known, but that it will be found among Ordovician crinoids is a 

 legitimate inference. 



