82 BULLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



chocrinus and Thaumatocrinus, though it is more probable that they are merely 

 cases of the radial assuming the characteristics of its subsequent reduplications, 

 the axillaries. 



It occasionally, though rarely, happens that individuals occur with six radials 

 and six postradial series. Such cases are usually sporadic, and instances have been 

 recorded in a number of widely separated groups. 



Among 340 specimens of Tropiometra picta from Rio de Janeiro which I 

 examined at the Museum of Comparative Zoology at Cambridge. Massachusetts, no 

 less than 17, or 6 per cent, were 6-rayed, though among the hundreds of specimens 

 of other species of this genus which I have studied I never found one with more 

 than five rays. 



These 6-rayed specimens are all but one of comparatively small size, the 

 diameter across the outstretched arms being between 100 mm. and 120 mm., the 

 exception having an expanse of 190 mm. and being the only one sexually mature; 

 normal specimens of this species average between 230 mm. and 270 mm. in expanse. 



An examination of the disks of 12 of the specimens (figs. 729-733, p. 346) 

 shows that in three cases it is quite impossible to determine which is the extra 

 ray, as there are six ambulacral groove trunks converging toward the mouth all 

 exactly alike; 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. 



In the Challenger report P. H. Carpenter mentions a small dry 6-rayed 

 " Antedon " in the collection of the British Museum. Suspecting that it was prob- 

 ably an example of this species, and also that it came from Brazil, I wrote to Prof. 

 F. J. Bell for information concerning it. He very kindly replied that it was, as 

 I had surmised, a Tropiometra, but that there was no record of the locality whence 

 it had come. 



These 6-rayed specimens of Tropiometra picta are 6-rayed throughout, possess- 

 ing a 6-rayed rosette and basal star (part 1, fig. 480, p. 363), in contrast to the 

 conditions found in Promachocrinus and in Tliaum-atocrinus^ where the rosette 

 and the basal star are both 5-rayed. 



Tropiometra picta is an extraordinarily variable form, its peculiarities being 

 such as to suggest that the very large proportion of 6-rayed indi^aduals is not 

 an inherent character of the species but is merely on a par with the relative number 

 of individuals aberrant in other equally noteworthy ways. 



Among the recent stalked crinoids the two species of the genus Rhizocrinus, 

 Eh. lofoten-sis and Rh. verriUi, are verj'^ variable in the number of their rays. 

 Among 7.5 specimens of Rh. lofotensis, Sars found found 43 with 5 rays, 15 with 

 4 rays, 1.5 with 6 rays, and 2 with 7 rays. In the related genus M onachocrinus 

 (fig. 179. p. 91, and part 1, fig. 122, p. 203) one species M. sexradi-atus (fig. 179, 

 p. 91), is only known with six rays, but of the species in the other living genera 

 of Bourgueticrinidsp, Bathycrlnufi, nycnmis. Bythocrinus (part 1, fig. 131, p. 203), 

 and Democrinus (part 1, fig. 133, p. 203), all appear to be 5-rayed without any 

 deviation, specific or individual. In M onachocrinus sexradiatus the basals are five 

 in number. In the two species of Rhizocrinus it has never been determined 



