438 DR. P. H. CARPENTER ON 



In three of the rays, therefore, there were six primitive joints, of which the second 

 and third became ultimately united by syzygy in two cases, but remained separate in a 

 third ; while in the other two rays there were primitively seven joints. 



The type, therefore, is intermediate between those more regular species, such as M. 

 angulatus from the Arafura Sea, with five primitive radials, two of which eventually 

 unite by syzygy, and the other group typified by 11. interruptus from among the Philip- 

 pine Islands, which have eight primitive radials that form two syzygial pairs. 



It is to the species last mentioned that M. rotundus presents, on the whole, the most 

 resemblance. There is about the same number of joints in the primary and secondary 

 arms, and the external characters of the stem are very similar in the two types. The 

 number of internodal joints is almost the same ; but the horizontal ridges on their surface 

 are much less marked in M. rotundus than in M. interruptus, while their outline is more 

 rounded and less sharply pentagonal (PI. LII. figs. 7, 5). 



But it is in the characters of the nodal and intranodal joints that M. rotundas differs 

 from M. interruptus, and still more so from all other species of the genus. 



In most Pentacrinidse the cirrus-facet is Limited to the nodal joint, but lies at the 

 broad end of a pear-shaped socket, the narrower part of which is continued downwards 

 on to the intranodal joint, and receives the base of the cirrus. The upper part of this 

 socket in Pentacrinus is confined to the nodal joint, not extending above the edge of the 

 cirrus-facet, which is often considerably lower than the vertical height of the nodal 

 joint; while the cirri are mostly directed downwards, and the infranodals are more or 

 less grooved to receive their bases, as described above. In Metacrinus, however, the 

 normal direction of the cirri seems to be upward rather than downward (Pis. L., LI., LII. 

 fig. 13) ; and the cirrus-sockets, therefore, encroach somewhat upon the supranodal, 

 which is thus slightly incised, as may be seen by comparing figs. 1 and 1 on PL LII. The 

 former represents a normal internodal joint, and the latter that immediately above the 

 node. The share of the supranodal joint in forming the cirrus-socket is likewise seen in 

 the side-view of the stem (PI. LII. figs. 5, 12, 18). In most species of Metacrinus the 

 intranodal is also slightly grooved to receive the cirrus-bases, and its upper (syzygial) 

 surface is, therefore, more or less stellate, in correspondence with that of the nodal joint 

 above it. This is least marked in M. interruptus, the nodal joints of which differ but little 

 in outline from those in other parts of the stem. The cirrus-facet ends below in a well 

 defined rim, which is distinctly above the lower edge of the joint. The syzygial surface, 

 therefore, has almost exactly the same pentagonal form as the ordinary sculptured face 

 of an internodal joint. That of the intranodal is similar to it, and has no reentering 

 angles as is usually the case. 



Now in M. rotundus there is the same well-marked termination of the cirrus-facet 

 above the lower edge of the nodal joint (PI. LII. fig. 5), and neither it nor the infra- 

 nodal is in any way grooved to receive the cirrus-bases (PL LII. fig. 3). Their apposed 

 surfaces, however, are not pentagonal, as in M. interruptus, but they are almost circular 

 (PL LII. fig. 3); and this character distinguishes the stem of M. rotundus from that of 

 all the other species of the genus. 



The type which approaches it most nearly in this respect is also a Japanese form, viz. 



