PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 21 



dorsal ridge, but this appears to be an optical effect due to a slightly increased density 

 of the calcareous deposit in this region. The texture of the segments of P! is finer, 

 more dense, and more uniform than that of the segments of the other pinnules so that 

 the segments appear smoother with a regularly and evenly convex dorsal surface. 

 The pinnules beyond Pj are composed of broad, thin, short segments which are not 

 more than half again as long as broad even in the slender distal pinnules, and are only 

 slightly, if at all, constricted centrally. The first two segments of the pinnules are not 

 broadened. 



The disk is completely enclosed in a continuous pavement of small delicate plates. 

 The ambulacral deposits in the pinnules, when present, consist of small and very 

 slender needlelike spicules. 



First of all it is necessary to determine whether Aporometra should be referred to 

 the Oligophreata or to the Macrophreata on the basis of the characters used in the 

 differentiation of these groups. These characters are given in volume 1, part 3, pp. 69 

 and 71. 



The cavity in the centrodorsal is moderately large, not so large as in most of the 

 Macrophreata, though larger than in the majority of the Oligophreata. Only the upper 

 half is occupied by the chambered organ and associated structures, the lower half being 

 occupied by a spongy calcareous filling. This secondary restriction of the volume of 

 the central cavity of the centrodorsal suggests relationship with the Oligophreata rather 

 than with the Macrophreata. 



The rosette is unusually well developed and is of the type characteristic of the 

 Oligophreata. 



The central portion of the radial pentagon is entirely open, with no calcareous 

 deposit, as in the Macrophreata; but this condition is also characteristic of certain 

 types in the Oligophreata, as for instance the Tropiometridae. 



The plane of the muscular fossae, although curving upward distally, makes a very 

 considerable angle with the dorsoventral axis, as in typical Oligophreata. 



The joint face elements of the articular faces of the radials distal to the transverse 

 ridge are moderately excavated, meeting in the median line at an angle of much more 

 than 90 as in the Oligophreata. 



The brachials are broader than long until far out on the arm as is characteristic 

 of most of the Oligophreata, in contrast to the majority of the Macrophreata. 



The syzygies are regularly and closely spaced, a characteristic feature of most small 

 comatulids with 10 arms, especially in the Macrophreata. The spacing of the three 

 earliest syzygies is, however, unique. The closest approach to it is seen in the very 

 small Microcomatula mortenseni (part 3, p. 287), belonging to the Oligophreata, in which 

 the syzygies are between brachials 3 + 4, 8 + 9, and 12 + 13, and distally at intervals of 

 2 muscular articulations. This distribution differs from that found in Aporometra 

 only in the position of the second syzygy, between brachials 8 + 9 instead of between 

 brachials 7 + 8. In the very small Antedon (formerly Compsometra) parnflora and. A 

 longicirra, belonging to the Macrophreata, syzygies occur between brachials 3+4, 

 9 + 10, and 14 + 15, and distally at intervals of 3 muscular Articulations. In view of the 

 general uniformity of the distribution of the syzygies hi the Macrophreata and the 

 variability in the Oligophreata, and in view of the fact that the nearest approach to 

 the conditions in Aporometra is found in Microcomatula, the spacing of the syzgies may 

 be regarded as suggesting relationship with the Oligophreata. 



