PART 5 A MONOGRAPH OF THE EXISTING CRENOIDS 23 



In some respects Aporometra bears a certain resemblance to some of the species 

 in the Mariametrida, and in others to some of the species in the Tropiometrida. Yet 

 the correspondence in no case amounts to identity of structure, and besides Aporometra 

 presents some features that are quite unique. Thus while Aporometra belongs to the 

 suborder Oligophreata it can not be assigned to any of the superfamilies within that 

 suborder. 



It remains to be seen whether any other comatulid type may be considered as 

 closely related to Aporometra. 



The kind of centrodorsal characteristic of Aporometra, with 10 columns of cirrus 

 sockets but with the surface not divided into radial areas, is found in Notocrinus. That 

 genus also has the pinnules with no trace of carination and composed of extraordinarily 

 short segments not longer than broad, and has a completely plated disk. The radial 

 articular faces of Aporometra and Notocrinus are in general similar, though differing in 

 details. Those of Aporometra are low and the elements on either side of the transverse 

 ridge lie in planes approximately at right angles to each other, while those of Notocrinus 

 are much higher and the elements on either side of the transverse ridge are in practically 

 the same plane. Both Aporometra and Notocrinus are viviparous. 



Dr. Torsten Gislen in 1924 made Notocrinus the type of a new subtribe, Noto- 

 crinida, of the Thalassometrida, which is essentially the equivalent of the Tropiometrida 

 as herein understood. The subtribe Notocrinida he divided into two sections, one 

 including forms with the side- and covering-plates moderately developed, the brachials 

 and pinnules rounded, and gonads in the arms the family Notocrinidae; and the 

 other including forms with the side- and covering-plates well developed, brachials and 

 pinnules prismatic (triangular in cross section), and gonads in the pinnules the family 

 Asterometridae (Asterometra and Pterometra}. 



The affinities of the family Asterometridae appear to the present author to be 

 wholly with the families Ptilometridae and Thalassometridae in the Tropiometrida, 

 and they may be omitted from further consideration in this connection. 



The most satisfactory disposition of the Notocrinida would seem to be to consider 

 it as a superfamily within the Oligophreata equivalent to and on the same basis as the 

 Comasterida, Mariametrida, and Tropiometrida, and to place within it two families, 

 Notocrinidae and Aporometridae, including Notocrinus and Aporometra respectively. 



[NOTE BY A.M.C.] In spite of the fact that two such experienced workers on 

 crinoids as Dr. Gislen and Mr. Clark have linked the family Notocrinidae, distinguished 

 by the extraordinary axillary gonads, with one or another family of comatulids with 

 gonads, as usual, on the pinnules, I cannot think that this is justified. The Aporo- 

 metridae appear to me to have no more in common with the Notocrinidae than they 

 do with any other comatulid family, in fact less if due weight is given to the difference 

 in the gonads. 



History. The genus Aporometra was established in 1938 b}' Dr. H. L. Clark who 

 gave a full diagnosis and listed Himerometra paedophora H. L. Clark as the type species. 

 In addition to Himerometra paedophora he included in the new genus Antcdon wilsoni 

 Bell and the new species Aporometra occidentalis. A ke}- Showing the interrelationships 

 of these three species was given. 



To this picture, no additions of note were made by Dr. H. L. Clark in his compre- 

 hensive work on Australian cchinodenns in 1946. 



