814 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



In both the Atelecrinidae and Pentametrocrinidae the arms are anomalous in 

 having lost features of fundamental phylogenetical significance common to all other 

 comatulids. The normal comatulid arm possesses IBr series, and extending from the 

 first to about the third syzygy there is a series of more or less oblong brachials which 

 are succeeded by nearly or quite triangular brachials. The IBr series represent a pair 

 of plates, one on either side of the original intersegmental boundary, which have come 

 to lie tandem instead of side by side on either side. The series of oblong brachials as 

 far as the third syzygy, represents the brachials of the pentacrinoid arm, with about 

 14 brachials developed and no pinnules. This pentacrinoid arm is not a real comatulid 

 arm, but represents merely a jointed appendage, more of the nature of a pinnule which, 

 however, at this point more or less abruptly changes its character and develops into 

 an arm. 



Since the arms of the Atelecrinidae have lost one of the two most important phylo- 

 genetic features of comatulid arms and since those of the Pentametrocrinidae have lost 

 the other feature, the arms in these two families are strictly comparable in being on the 

 same phylogenetic plane, which is in advance of that of the arms in any of the other 

 families. Since the arms in these two families are on a strictly comparable plane, it is 

 not surprising that the type peculiar to one should reappear in the other. 



The arms of Atopocrinus, it should be noticed, combine the loss of the IBr series 

 otherwise peculiar to the Pentametrocrinidae with the loss of the earlier oblong 

 brachials characteristic of the Atelecrinidae. They are, therefore, the most highly 

 specialized arms to be found among the comatulids. 



In the families Comasteridae, Colobometridae, Antedonidae (Zenometrinae and 

 Perometrinae), and Pentametrocrinidae, species (or genera) with deficient and also with 

 complete pinnulation occur. In nearly all cases the species with deficient pinnulation 

 have the arm bases so closely crowded as to prevent the proper functioning of the 

 earlier pinnules, which are therefore lost. There is, therefore, nothing remarkable in 

 the reappearance of all the pinnules in a genus of Atelecrinidae in which the arm bases 

 are not crowded as they are in Atelecrinus itself. 



Since the differences between Atopocrinus and Atelecrinus are of relatively small 

 importance, while the differences between Atopocrinus and all the other comatulid 

 genera are fundamental, there appears to be sufficient justification for placing 

 Atopocrinus in the Atelecrinidae. 



ATOPOCRINUS SIBOGAE A. H. Clark 



[See vol. 1, pt, 1, fig. 227, p. 245] 



Atopocrinus sibogae A. H. CLARK, Notes Leyden Mus., vol. 34, 1912, p. 151 (description; Siboga sta. 

 177); Unstalked crinoids of the Siboga-Exped., 1918, p. 263 (detailed description; sta. 177), p. 274 

 (listed); fig. 15, p. 263; pi. 26, fig. 95. GISLEN, Zool. Bidrag Uppsala, vol. 9, 1924, p. 26 (new 

 information regarding the pinnulation), p. 83 (articulations). 



Description. The centrodorsal is elongate conical, 5.5 mm. broad at the base and 

 7 mm. long, with the tip slightly blunted and the sides straight. Five strong interradial 

 ridges, each about as broad as the adjacent columns of cirrus sockets, divide the surface 

 of the centrodorsal into 5 radial areas, each of which is bisected by a narrow median 

 ridge which, except at the base, is as high as the interradial ridges. 



The distal (dorsal) border of each cirrus socket is produced outward, forming a 

 strong ridge across the proximal border of the one next below it. Thus each cirrus 

 socket occupies an approximately oblong rather deep pit, bounded proximally and 



