20 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



[Note by A.M.C] It is noteworthy that Aporometra is the only viviparous coma- 

 tuHil genus not known from the Antarctic. Isometra and Phrixometra extend up the 

 coast of Argentina in deep water but are predominantly Antarctic genera. 



Systematic position— The systematic position of the genus Aporometra has not 

 heretofore been satisfactorily determined. The first species to be described {nnlsoni) 

 was said to be the representative on the south coast of Australia of Perometra pusilla, 

 a species belonging to the subfamily Peromctrinae of the family Antedonidae. The 

 second species (paedophora) was described as a species of Ilimerometra. Both these 

 species were regarded by the senior author as the young of Ptilometra. These three 

 genera are about as widely separated as they could be systematically. 



Dr. H. L. Clark suggested that the status of his Ilimerometra paedophora and of BeU's 

 Antedon unlsoni should be reexamined on the basis of numerous specimens of a similar 

 kind tiiat he had collected in Western Australia in 1929. He was so very kind as to 

 send me all his specimens from Western Australia, together with his specimens of 

 Ilimerometra paedophora and two syntypes of Antedon wilsoni borrowed from the British 

 Museum through the courtesy of Dr. D. Dihvyn John. 



Witli practically" all the known specimens referable to species of Aporometra at 

 hand, the question of its systematic position was examined in the greatest detail. 



The necessary dissections were made on the specimens from Western Australia 

 (Koombana Bay) with which it was assumed the others would agree. 



An analj'sis of the characters presented by these specimens, and the conclusions 

 reached through the study of these characters, follows. 



The diameter of the central cavity in the centrodorsal is approximately a third of 

 the diameter of the centrodorsal at the same level. The central cavity reaches the 

 apex of the centrodorsal, but its lower half is separated from the upper half by a deli- 

 cate partition having the upper surface concave, beneath which is a loose spongy filling. 

 Tlie longitudinal ridges on the sides of the central cavity are unusually high. Shallow 

 radial pits are present on the ventral surface of the centrodorsal. 



The rosette is highly developed, with both the radial and interradial extensions 

 forming "spoutlike processes." It is relatively thick and is countersunk within the 

 radial circlet. 



The central portion of the radial pentagon is free of calcareous deposits. 



The plane of the elements of the joint faces of the radials distal to the transverse 

 ridge is at right angles to the dorsoventral a.xis and approximately at right angles to 

 tlic plane of the dorsal ligament fossa, which is parallel to the dorsoventral axis. In 

 other words, when the radial ring is viewed ventrally the joint face elements distal to 

 the transverse ridges are at right angles to the du-ection of vision, while the dorsal 

 ligament fossae, lying in a plane parallel to the dbection of vision, are not visible. 

 The muscular fossae are small and moderately excavated. The ligament pits are 

 slightly developed. 



The cirrus sockets are arranged in 10 regular columns. 



The brachials from about the fourteenth onward are broader than long, remaining 

 broader than long for some distance out on the arm. 



The pro.ximal brachial syzygies are between brachials 3+4, 7 + 8, and 12 + 13, 

 and the distal intersyzygiul interval is usually 2, sometimes 3, muscular articulations. 



The pinnules are rounded dorsally and are not prismatic. Sometimes when dry 

 the lower segments of the earlier pinnules give the impression of having a fine mid- 



