MONOGRAPH OF THE EXISTING CRINOIDS. 



129 



Three somewhat anomalous departures from the usual type of brachial struc- 

 ture deserve to be noticed here. 



In AteJecr'mus (part 1, figs. 123, p. 192, and 124, 125, p. 193) and in Atopo- 

 ci'luus (part 1. fisr. 227, p. 2-15) the series of wedge-shaped or more or less oblong 

 brachials which typicall_v intervenes between the first syzygial pair and the triangu- 

 lar brachials beyond the second syzygy is 

 absent, and the brachials assume the tri- 

 angular form immediatelj^ bej'ond the 

 second. 



In the Penf(nnetrocrinid<j> (part 1, figs. 

 113, p 181, 119, p. 185) and in Atopocrinus 

 the first syzygial pair is separated from the 

 first brachial pair by a single brachial. 



Both of these departures from the nor- 

 mal type of brachial structure are probably 

 correlated with the excessive length and con- 

 sequently very rapid growth of the arms. 



In most of the species of Crosso- 

 metra, Perissomefra, and Pachi/Iomefra the 

 eleventh, twelfth, or thirteenth brachial is 

 usually sharply differentiated from the 

 brachials preceding and succeeding by be- 

 ing regularly oblong with its proximal and 

 distal articular faces parallel to each other ; 

 the transverse ridges on these articular 

 faces, however, alternate as usual. 



BRACHIALS OF THE ARSIS OF THE MULTIBRA 

 SPECIES. 



-Lateral view of specimen 

 cotvlometea obnata. 



In the multibrachiate comatulids (figs. 

 241, 243, p. 197, 244-252. p. 199 and 253-261, 

 p. 205) the free undivided arms differ from 

 those of the 10-armed species (figs. 23&-240 

 [also 242], p. 197) and the 10-armed young 

 of the multibrachiate types in being com- 



. . Fig 



posed of shorter brachials which have the 



ends less oblique and are consequently 



wedge-shaped instead of triangular, and in having the syzygial pairs less numerous 



and less regularly distributed. 



Speaking broadly, it may be said that a species with 20 arms has the brachials 

 shorter than one with 10 arms, and wedge-shaped instead of triangular; a species 

 with 40 arms has the brachials still shorter and less obliquely wedge-shaped ; and a 

 species with more than 60 arms has this character still more pronounced. But 

 while all the multibrachiate species have the brachials shorter and more wedge- 



