734 MR. P. H. CARPENTER ON THE [ Dec. 19, 
joints which become immovably united and behave in all respects as 
one. But it may be united by syzygy to the second radial, instead 
of by the usual bifascial articulation'. This, however, is of an alto- 
gether different morphological value from the syzygial unions of the 
arm-joints. In the latter case the hypozygal entirely loses its indi- 
viduality as a separate joint, and bears no pinnule as the epizygal 
and the remaining brachials do. Thus, for example, in very nearly 
all Comatule the original third and fourth joints of the growing arm 
differ from those which ultimately appear beyond them. For 
‘whilst the majority of these gradually come to possess the true 
articulations, and to be separated by the intervention of muscles and 
ligaments, a certain small proportion become more intimately united 
on a simpler plan, which admits of no motion between them”. The 
double or syzygial joints thus formed resemble the ordinary brachials 
in bearing but one pinnule, and they are therefore best considered as 
single joints. In Ant. rosacea, for example, the 3rd and 4th, the 9th 
and 10th, and the 14th and 15th joints of the growing arm are 
respectively united in pairs by syzygy; but the arm is best described 
as having syzygies in the 3rd, 8th, and 12th joints. So again in the 
numerous Comatule, such as Act. parvicirra, which have axillaries 
on some or all of the primary arms. Counting from the third radial, 
the distichal axillary is primitively the fourth joint. he first, as is 
almost invariably the case, bears no pinnule, while the second does 
bear a pinnule, but the third not; for it is united to the followin 
(axillary) joint by a syzygy. The first ray-division would therefore 
be described as consisting of three distichal joints, the second bearing 
a pinnule, and the third (axillary) being a syzygy. 
In Encrinus, in most recent species of Pentacrinus, in a few Coma- 
tule (Act. solaris, &c.) the two outer radials and the first two joints 
beyond them are respectively united by syzygy ; and on the principle 
explained above, each pair would be considered as forming a single 
joint, so that the true third brachial (itself a syzygial joint) would 
come to be the second. This would involve our describing these 
forms as having but two radials, the axillary with a syzygy, and 
syzygies both in the first and in the second brachials. I think, 
however, that this would be misleading and make the difference be- 
tween the two types appear much greater than it really is. 
The presence of three radials is such an absolutely constant 
character in all the five-rayed Neocrinoids, excepting Metacrinus & 
Plicatocrinus, that the fact of the two outer ones being united by 
syzygy and not articulated seems to me to be of minor importance ; 
and I do not assign to it the same morphological value as the 
syzygial union of the third and fourth primitive brachials, in which 
* I use this name to denote the presence of a ligamentous bundle on either 
side of a vertical articular ridge, while muscles are absent. In by far the greater 
number of Comatule this is the mode of union of the two outer radials and 
of the first two joints beyond each axillary. It also occurs in Miéllericrinus, 
Apiocrinus, Pentacrinus decorus, P. blakei and other types. But in Bathycrinus 
there isa trifascial articulation, a small additional bundle of fibres being inserted 
ee the apposed faces of the joints at the lower or dorsal end of the articular 
riage. 
P W. B. Carpenter, Phil. Trans. 1866, p. 721. 
