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REPORT ON THE CRINOIDEA. 287 
arm-fragments belong. But as I have pointed out above, the peculiarities of the arm- 
grooves upon the ventral surface of the skeleton, and their relation to the ambulacra, 
afford characters of considerable systematic value. The joints of Pentacrinus naresianus, 
however (Pls. XXVIII, XXIX.; Pl. XXX. figs. 1, 23), show a distinct indication of the 
more oblique shape which is common among the Comatule; while both in this species 
and in Pentacrinus blakei the peculiar nature of the syzygial union renders the arms 
very readily distinguishable (Pl. XXX. fig. 23; Pl. XXXa. figs. 9-12; Pl. XXXI. 
figs. 1; 25) PEON figah 53/7012; 14): 
The characters of the stem of the Pentacrinidee have already been fully discussed 
(ante, pp. 12-23). The fact that during growth it undergoes rather considerable modifi- 
cations in its appearance has led to a very general belief in the impossibility of identifying 
species by means of stem-fragments only. This is more especially the case as regards 
the fossil species, which are often based upon the stem-characters alone, since calyces 
are but rarely:met with; and I think it not improbable that two or even more 
types of stem from the same horizon, to which different specific names have been given, 
may sometimes be only different stages of growth of one and the same species. Thus, 
for example, four species might be made out of the joints represented in Pl. XXII. 
figs. 13, 14, 23, and 26, which are, however, merely different stages of growth in the 
stem of Pentacrinus wyville-thomsoni. But on the other hand, the difference between 
the stem-fragments figured on PI. XIII. fig. 8 (Pentacrinus asterius) and Pl. XIX. fig. 4 
(Pentacrinus wyville-thomsoni) are obviously not due to any developmental causes ; 
and the two would be universally recognised as belonging to different species, even 
if nothing whatever were known about the calyces and arms belonging to them. 
The same remark holds good in the case of Metacrinus, isolated stem-fragments of 
Metacrinus alternatus (Pl. XXXIX, fig. 3), Metacrinus cingulatus (Pl. XLI. fig. 1), and 
Metacrinus nobilis (Pl. XLI. fig. 5), to say nothing of other species, being very 
readily recognisable. 
What has been written above refers simply to the general appearance of the stem- 
fragments ; but when the number of internodal joints is taken into consideration, and also 
the markings on their terminal faces, the characters of the stem as'a whole must be 
regarded as of very considerable systematic value. The stem-joints of Balanocrinus and 
Extracrinus are very readily identified by the sculpture on their faces; but as far as the 
internodal joints are concerned, I am unable to find any constant difference in this respect 
between Pentacrinus and Metacrinus. In most (recent) species of both genera there are 
from three to six strongly marked ridges along the sides of each petaloid figure. The 
proximal ridges joi their fellows in each interpetaloid space, while the outer ones reach 
the exterior and cause the crenulation along the line of union between every two joints, 
the ridges of each face alternating in position with those of the face opposite to it 
(Pl. XIII. figs. 10, 11; Pl. XXII. figs. 22-24; Pl. XXVI. fig. 17; Pl. XXX. figs. 25-30; 
