244 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Note on six-rayed specimens of Tropiometra oarinata (Lam.). 



Six-rayed individuals of recent free crinoids have hitherto been regarded as 

 quite rare. Although tetraradiate examples are not uncommon, I can find but a 

 single record of a specimen with more than five radials. It was therefore with 

 considerable surprise that I found among about three hundred and forty speci- 

 mens of Tropiometra carinata no less than seventeen. It is interesting that all of 

 the six-rayed specimens came from Rio Janeiro, all of the sixty or more from 

 Zanzibar and Mauritius being normal* This gives us for the Brazilian specimens 

 6 % of six-rayed individuals. 



These six-rayed specimens are all but one of comparatively small size, the 

 diameter being between 100 mm. and 120 mm., the exception having an expanse 

 of 190 mm. ; this last is the only one sexually mature. Normal specimens of this 

 species average from 230 mm. to 270 mm. in diameter, the size of those from Rio, 

 Zanzibar, and the south Pacific being practically the same. 



An examination of the disks of twelve of the specimens shows that in three 

 cases it is quite impossible to determine which is the extra ray, as there are six 

 ambulacra converging on the disk, all precisely alike ; an examination of the rays 

 themselves also furnishes no clue ; one specimen has the interpolated ray between 

 the two on the left side, one has it behind the right posterior, while seven have 

 the extra ray inserted behind the left posterior. 



Dr. Carpenter, in his monograph on the " Comatulae " collected by the " Chal- 

 lenger," mentions a small dry six-rayed " Antedon " in the British Museum 

 collection. Suspecting from my examination of these specimens that it was prob- 

 ably an example of the same species, and also from Brazil, I wrote to Professor 

 Bell of the British Museum for information concerning it. He very kindly replied 

 that it was, as I had surmised, Tropiometra carinata, but there was no record of 

 the locality whence it had come. 



In the recent stalked crinoids it is interesting to note that Rhizocrinus lofotensis 

 alone is known with more than five rays, and, as in Tropiometra carinata, this 

 variation is confined to a single locality, the coast of Norway. 



Among the fossil crinoids six-rayed individuals appear to be extremely rare, the 

 figure by Rosinus (De stellis marinis quondam nunc fossilibus, p. 24, no. 3, pi. 1, 

 fig. 3, 1719) of a six-rayed Encrinus liliiformis being the only record I know of 

 this condition. 



The genera used for the free crinoids in this paper are those recently proposed 

 in a preliminary paper on a revision of the family Antedonidae (sensu A. H. 

 Clark, 1907), in which that family is restricted so as to be equivalent to the 

 genus Antedon, as understood by Dr. P. H. Carpenter. The old genus Antedon 

 is broken up into a number of well-marked homogeneous genera, whereby the inter- 

 relations of the various species are much better shown than by the old method of 

 uniting some three hundred or more widely varying specific types under one 

 generic name. The following key shows the relations of these genera to each 

 other from the point of view of differential characters. There are, in addition 



