812 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



Remarks [by A.M.C.]. — Mr. Clark left no diagnosis or discussion of this family 

 since his Siboga report revealed the existence of the genus Atopocrinus and of the new 

 species Atelecrinus anomaly s, for which he has here created the new genus Sibogacrinus. 

 The characters of these two genera necessitate considerable qualification of the diagnosis 

 previously given for Atelecrinus alone. Of the three main characters which formerly 

 marked off the Atelecrinidae from other comatulids, the visibility of the basals in the 

 adult and the absence of proximal pinnules arc not sliared by Atopocrinus and the ele- 

 vations round the cirrus sockets are absent in Sibogacrinus, which on the other hand 

 does have the basals prominent, even more so than in Atelecrinus. Unfortunately, the 

 unique holotype of Sibogacrinus anomalus has lost the anus from the first syzygy, so 

 that the occurrence of its pinnules is unknown. 



With the inclusion of Atopocrinus in the family, the Atelecrinidae shows an inter- 

 esting parallel to the Pentametrocrinidae with the diflerence that its lO-armed genera, 

 Atelecrinus and Sibogacrinus, have division series but in Thaumatocrinus of the other 

 family the duplication is from the radials. 



In Atelecrinus and probably also in Sibogacrinus there appears to have been a 

 neotenous tendency with regard to the nondevelopment of the proximal pinnules, 

 which normally appear in the ontogeny of most comatulids after the distal pinnules 

 have been formed. 



I am not entirely convinced that Atopocrinus is so closely related to Atelecrinus 

 as Mr. Clark suggests. Since he has separated the 5-armed oligophreate genus Eudio- 

 crinus with no division series as a family distinct from the Zygometridae, where 

 such series are present, I am inclined to think that the parallel distinction of Atopocrinus 

 from the rest of the Atelecrinidae is warranted. However, I agree that Atopocrinus 

 seems to be more closely related to the Atelecrinidae than to any other comatulids. 

 The characters provided by the centrodorsal particularly are undeniably similar. 



KEY TO THE GENERA OF ATELECRINIDAE 



a>. Five undivided arms; no IBr series; first syzygy between brachials 4+5; no basals (Moluccas; 



1633 meters) Atopocrinus (p. 812) 



a'. Ten arms; IBr series present; first syzygy between brachials 3 + 4; basals present. 



6'. Basals small, forming low triangles in the interradial angles the attenuated lateral angles 

 of which may or may not come into contact beneath the radials; cirrus sockets flanked 

 on either side by a high elevation triangular in profile, the two elevations on either side 

 of a socket sometimes joined about the proximal (ventral) border of the socket by a low 

 rim, forming with this a horseshoe-shaped ventrolateral rim highest at the two ends; cirri 

 very delicate and slender, very gently curved, composed of more than 20 segments of which 

 the distalmost are about twice as long as broad; terminal claw minute and conical (Hawai- 

 ian Islands to Fiji and the Moluccas; Brazil to Florida; western coast of Ireland; 532 [?384]- 

 1256 [71479] meters) Atelecrinus (p. 817) 



h'. Basals very large, the height equal to more than half the width, and broadly in contact 

 laterally; surface of the centrodorsal smooth, with no elevations flanking the cirrus sock- 

 ets; cirri recurved, with not more than 20 segments of which those in the outer third are 

 much shorter than the proximal, and but little longer than broad; terminal claw long, 

 stout, and recurved (Moluccas; 1633 meters) Sibogacrinus (p. 832) 



Genus ATOPOCRINUS A. H. Clark 



Atopocrinus A. H. Clark, Notes liCyden Mus., vol. 34, 1912, p. 150 (diagnosis; type species A. 

 sihogae); Unstalked crinoids of the Siboga-Fjxped., 1918, p. viii (discovery by the Siboga), p. 262 

 (in key). — Gisl£n, Zool. Bidrag Uppsala, vol. 9, 1924, pp. 26 and following (discussion); Ark. 



