160 THE CRINOIDEA 



iBr in the dorsal cup (becoming " fixed brachials," Br), while the corre- 

 sponding Ainb are either incorporated in, or pressed below, the tegmeii 

 by iAmb ; all thecal plates united by suture, somewhat loose in the 

 earliest forms, but speedily becoming close, and producing a rigid theca ; 

 mouth and tegminal food-grooves closed ; arms pinnulate. 



The families may be grouped somewhat after the plan of Wachsmuth 

 and Springer (1897), thus : 



A. No anal plate in radial circlet of patina. BB usually form a 



pentagon. Melocrinoidea. 



B. An anal plate between RR in post. IR. BB form a hexagon. 



1. Proximal anal heptagonal, succeeded by one or more in the 



same vertical series, between the ordinary iBr. Batocrin- 

 oidea. 



2. Proximal anal hexagonal, succeeded by no vertical series, but 



by 2 iBr. Actinocrinoidea. 



Whether Group B. was derived from some genus in Group A. is un- 

 certain ; representatives of both groups are found in the Ordovician. It 

 is more probable that .5,2 was derived from J5,l, by way of the Periecho- 

 crinidae. Among Melocrinoidea the Patelliocrinidae are scarcely removed 

 from the Inadunata, and some of their genera might almost be placed 

 with the early Adunata, with which they were contemporary. Although 

 their occurrence in the Ordovician is doubtful, yet the existence of the 

 family points to the path along which the Camerata ascended. The 

 Glyptocrinidae and Melocrinidae, which differ in little but number of 

 BB, and both have anals in the dorsal cup, may have been derived from 

 such a form as Stelidiocrinus, which also has anals. With less doubt we 

 infer that the Patelliocrinidae without anals gave rise to the similarly 

 constituted Clonocrinidae, from which sprang Eucalyptocrinidae, and 

 probably also Dolatocrinidae. Among Batocrinoidea the simplest and 

 one of the oldest genera is Tanaocrinus, with 5 BB, and not far from 

 it come the earlier Xenocrinidae, with 4 BB, which perhaps led on to 

 Periechocrinidae, and so to Actinocriuoidea; The important ancestral 

 family, however, is the Silurian Carpocrinidae, in which Acacocrinus is 

 nearest to the Inadunate type. From them arose Barrandeocrinus with 

 its recumbent arms, then the Coeliocrinidae without anal tube or respira- 

 tory pores, and, later on, the Batocrinidae possessed of both those structures. 

 No members of this order survived the Lower Carboniferous, but during 

 their history they developed some of the most numerous in individuals 

 and species among crinoid genera, and in Barrandeocrinus, Eucalyptocrinus, 

 Agaricocrinus, and Strotocrinus, some of the most remarkable of all 

 Echinoderma. 



SOB-ORDER 1. Melocrinoidea. Monocyclica Camerata with RR in 

 contact all round ; IBr x usually quadrangular. 



FAMILY 1. GLYPTOCRINIDAE. Melocrinoidea with 5 BB ; in each half- 

 ray 2-8 HBr, sometimes IliBr ; free arms rarely branch beyond IIIBr, 

 and may be uniserial, zigzag, or biserial ; iBr numerous but definite ; 

 illBr numerous, less definite ; a ridge of anals in post. IR ; tegmen 

 of numerous small plates ; stem round, with pentagonal lumen. All 

 Ordovician of N. America, European representatives doubtful. Genera 



