A MONOGRAPH OF THE EXISTING CKINOIDS 17 



thus Rhizocrinus lofotensis and the corresponding species on the American coast 

 are very commoiily 6-rayed, while the numerous species of the same genus within 

 the normal tropical area are invariably 5-rayed, and the most variable species of 

 Metacrinus, M. rotundus, is also the most northerly of the genus; it is the same with 

 species; the examples taken farthest from the center of distribution, or at the limits 

 of distribution, are the most variable; Tropiometra carinaia, constant in its char- 

 acters from east Africa to Oceania, is extremely variable in Brazil; Antedon bifida 

 is more variable about the British coasts than in the Mediterranean, and Heliometra 

 eschrichtii varies most along the southern limit of its range. 



In a paper published on June 16, 1908, some cases of abnormal arm structure 

 in the crinoids are described. 



A 6-armed specimen of Rhizocrinus lofotensis from off the southern United 

 States is recorded. 



The only specimen of Poecilometra acoela dredged by the Albatross off southern 

 Japan has a third costal (IBr element) inserted between the normal two on one 

 of the rays, as was the case in a specimen of Thaumatometra alternata dredged by 

 the Challenger just north of New Gmnea. The use of the name Poecilometra acoela 

 for the specimen described as the type of Antedon [Poecilometra] scalaris indicates 

 that the latter is now considered a synonym of the former. 



Mr. Frank Springer in his monograph on Uintacrinus figured a specimen of U. 

 socialis with four costals on one ray, an additional pair being inserted between the 

 normal costals and the first brachials. From the shape and proportions of these 

 additional costals it is stated that one might almost infer that they were united by 

 syzygy, in which case the costal series would be directly comparable to the distichal 

 (IIBr) series in comatulids with 4 (3 + 4) distichals. The presence of 4 (3+4) costals 

 has only once been detected, but there is no reason to believe that it does not more 

 or less commonly occur. 



A case of distal arm division in a specimen of Himerometra stylifer from Kago- 

 shima Bay, Japan (obtained by the United States Exploring Expedition and not 

 previously recorded) is described, and the difference between this type of arm branch- 

 ing and that occurring at the base of the arms is discussed. 



An instance of three succeeding proximal pinnules on the same side of the arm in 

 Charitometra imbricata (new name) { = Antedon granulifera P. H. Carpenter, 1888, not 

 Antedon granulifera Pourtales, 1878 = Comatula brevipinna Pourtales, 1869= Antedon 

 pourtalesi P. H. Carpenter, 1888) is given. 



The reasons for the use of the new name imbricaia are not given further, but they 

 were as follows. 



Pourtales originally bestowed the name brevipinna upon a young specimen of a 

 species of Crinometra on which "one of the arms is abortive and divided into three 

 very short branches; to compensate, one of the arms of the next pair is divided into 

 two from its origin;" in other words, it was undergoing adolescent autotomy. 



In the Challenger report pourtalesi is diagnosed by Carpenter as follows: IIBr 2; 

 "15-20 stout cirrus joints; cirri without definite arrangement; the distichals and 

 lower brachials have distinctly flattened sides; the later cirrus joints smooth; calyx 

 and arm bases irregularly tubercular; the pinnules from the tenth to the twentieth 

 brachials have the third-fifth joints flattened and expanded laterally." 



