X28 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



Comaster marice and in a species mentioned, but not named, by Car- 

 penter (of which I have been able to examine specimens), it is very 

 irregular, and in Isocrinus blakei, decorus, and asteria it is usually 

 more or less, and sometimes very, irregular, especially in the last 

 named. In all the species in which extraneous division occurs, the 

 irregularity increases with each successive arm division, so that, in 

 Metacrinus and in Isocrinus asteria, with their numerous division 

 series, the later division series are of very numerous joints, and 

 much more variable than the division series of /. blakei and /. de- 

 corus, whose most distal series correspond to one of the more proximal 

 series of /. asteria and Metacrinus. 



Metacrinus (fig. 28). — The species of Metacrinus are remarkable 

 in possessing a type of arm structure different from any we have 

 considered. There are no synarthries in the Metacrinus arm; the 

 first two post-radial joints are always united by syzygy (the second 

 bearing a pinnule), the second and third by an oblique muscular 

 articulation; all the subsequent articulations are oblique muscular, 

 with the exception of occasional syzygies. Therefore Z x and Z 2 are 

 recognized as the first two post-radial joints, occupying the same 

 position in which we found them in Pentametrocrinus, Decametro- 

 crinus, and the peculiar Thaumatocrinus / but while in these genera 

 the arms are undivided, in Metacrinus, extraneous division always 

 occurs, often as many as five times. All the axillaries in Metacrinus, 

 therefore, always have the proximal and both distal faces oblique 

 muscular, while in Isocrinus blakei, decorus, and asteria, the first 

 post-radial axillary has straight muscular faces, distally, synarthrial 

 proximally, the remainder all oblique muscular; and in /. wyville- 

 thomsoni, I. parrce, I. alternicirrus, and /. sibogce all the axillaries 

 have distal faces with straight muscular articulations and proximal 

 with syzygial. Now, in the young stages of most of the comatulids 

 and in the genus Isocrinus where the adults are multibrachiate, the 

 young have only ten arms, Z t being separated from the radials by a 

 single interpolated series, representing an additional Z x and Z L> ; in 

 adult life, Z 2 is, in most multibrachiate comatulids, and in Iso- 

 crinus wy ville-thomsoni, I. parrce, I. altcrnicirrus, and /. sibogce, 

 separated from the radials by a number of interpolated division se- 

 ries; in Comaster (as restricted) and in Isocrinus blakei, decorus, 

 and asteria, Z : remains in its primitive position, while extraneous 

 division occurs beyond it ; but in Metacrinus Z x is always the first 

 post- radial joint, and is never separated from the radial by an inter- 

 polated series. This is interesting; for the ten-armed young stage 

 of multibrachiate forms depends on the presence of a single inter- 

 polated series, and, as this series (which invariably persists in after 

 life) is absent in Metacrinus, the natural inference is that Meta- 



a Challenger Reports, XXVI, Zoology, p. 328. 



