12 THE VOYAGE OF H.M.S. CHALLENGER. 



Guettardicrinus, and in the majority of those of Millericrinus ; and yet it is considered by 

 Wachsmuth and Springer as a merely " accidental " occurrence, and the real symmetry of 

 the top stem-joint in the Apiocrinidse is described as interradial. It is actually and visibly 

 so in some twenty species of Millericrinus. But they belong to that aberrant section of the 

 genus which so closely approaches Pentacrinus in having a distinctly pentagonal stem with 

 interradial angles, and articular faces the sculpture of which is very different from that of 

 the typical Apiocrinidse and somewhat closely resembles that of the joint-faces in certain 

 Pentacrinidae. In all of these species the top stem-joint, like those below it, has interradial 

 angles, and the same is the case with the basal concavity into which it fits. But Wachs- 

 muth and Springer tell us that the " natural " shape of this concavity in the Apiocrinidse 

 is to have radial angles, and they have not attempted to explain its interradial symmetry 

 in these aberrant and Pentacrinus-l\ke forms of Millericrinus by reference to any causes 

 whatever, accidental or otherwise. Perhaps it has escaped their notice ; but whether 

 this be the case or not, it is somewhat surprising to students of the Neocrinoidea to be 

 told that the distinctive characters of the top stem-joint in the Apiocrinidse, presenting 

 themselves in each of the three genera, and in by far the greater number of the species of 

 this family, are due to " accidental " causes. Further discussion of this question, however, 

 would be impracticable at present. I merely wish to point out that as soon as the 

 centro-dorsal of the early larva of Comatula takes a definite shape its angles are distinctly 

 radial, just as is permanently the case in the top stem-joint of Apiocrinus, and this is 

 in itself an argument against the supposed change of symmetry in the latter type about 

 which Wachsmuth and Springer write so positively. But when the cirri appear on the 

 centro-dorsal and the basals begin to be transformed into the rosette, the outline of the 

 centro-dorsal changes. The basals are no longer the principal plates in the calyx, but 

 they undergo metamorphosis into the small rosette, and the centro-dorsal increases rapidly 

 in size, more so than any other part of the skeleton, " so that it soon comes to pass beyond 

 the circlet of basals, and to abut on the proximal edge of the first radials ; and instead of 

 stopping here it continues to increase in diameter until it conceals the whole inferior 

 surface of the first radials, and sometimes even encroaches somewhat on the second." l 



Here then we see the reason for the interradial angles of the centro-dorsal in the 

 mature Comatula. It is an altogether secondary condition, and due to the fact that the 

 fossae on the ventral surface of the centro-dorsal lodge the radial plates, so that the ridges 

 separating them are interradial, just as in Apiocrinus the fossa? on the top stem -joint 

 lodge the basals and are interradial, so that the intervening ridges and the angles in which 

 they terminate are radial. Even in fossil Comatula? which have no rosette, but persistent 

 basals, these plates are usually quite small and do not form a closed ring on the exterior 

 of the calyx; so that the upper surface of the centro-dorsal is mainly occupied by the 

 radial fossa? and has interradial angles as in recent Comatula? (PI. I. figs. 5, 6a, 8d; PI. II. 



1 W. B. Carpenter, Phil. Trans., 1866, p. 742. 



