MONOGRAPH OF THE EXISTING CRINOIDS. 



129 



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

 ture deserve to be noticed here. 



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

 crinus (part 1, fig. 227, p. 245) the series of wedge-shaped or more or less oblong 

 brachials which typically 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 immediately beyond the 

 second. 



In the Pentametrocrinidw (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- 

 nietra, Penssometra, and Pachylometra 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 ARMS OF THE MULTIBRACHIATE 

 SPECIES. 



FIG. 



199 



199. LATERAL VIEW OF SPECIMEN OF 

 COTYLOMETBA 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. 236-240 

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

 of the multibrachiate types in being com- 

 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- 



