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 (Pis. XXVIIL, XXIX.; PL XXX. figs. 1, 23), show a distinct indication of the 

 more oblique shape which is common among the Comatulge; while both in this species 

 and in Pentacrinus hlakei the peculiar nature of the syzygial union renders the arms 

 very readily distinguishable (PI. XXX. fig. 23; PI. XXXa. figs. 9-12; PI. XXXI. 

 figs. 1, 2 ; PL XXXII. figs. 5, 7, 12, 14). 



The characters of the stem of the Pentacrinidse 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, wdiich are, however, merely different stages of growth in the 

 stem of Pentacrinus loyville-thomsoni. But on the other hand, the difference between 

 the stem-fragments figured on PL 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 cingxdatus (PL XLI. fig. 1), and 

 Metacrinus nohilis (PL XLI. fig. 5), to say notliing 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 join 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 XIIL figs. 10, 11 ; PL XXII. figs. 22-24 ; PL XXVI. fig. 17 ; PL XXX. figs. 25-30; 



