50 THE VOYAGE OF H.M.S. CHALLENGER. 



the extent of the arm-divisions. Individuals with a simdar distichal axillary on each 

 primary arm and no further division, so that the number of arms is exactly twenty, are 

 extremely rare, except in Actinometra paucicirra, and to a less degree also in Actino- 

 metra pulchella. Another Caribbean species {Antedon spinifera) not unfrequently has 

 exactly thirty arms, owing to the very regular presence of palmar axillaries upon the 

 inner pair of every four secondary arms. But I cannot call to mind any species of 

 Comatula among the many hundred forms which I have examined in which the total 

 number of arms is exactly forty, owing to the presence of ten distichal series and twenty 

 series of palmars. I have seen an Actinometra parvicirra with thirty-nine arms, an 

 Antedon articulata with forty, and Bell's unique specimen of Antedon gyges has forty- 

 one. But I do not remember any species which always has exactly forty, and I doubt 

 if there be one ; while I can say with tolerable confidence that no one wdl ever find a 

 specific type which always has ten distichal axillaries, twenty palmars, and forty post- 

 palmars, thus giving rise to exactly eighty arms. The logical result of Bell's use of 

 brackets therefore would be that every Comatula with eleven to nineteen arms should 

 have the symbol for the distichals placed between brackets ; for those with twenty-one 

 to thirty-nine arms there should be brackets round the palmar sign and generally also 

 round the distichal one as well ; while the formulae of types which have over forty and 

 less than eighty arms should have the last, if not the two last, symbols within brackets. 



A reference to Bell's formulae 1 for Antedon articulata and Antedon gyges, and to 

 those for Actinometra alternans, Actinometra parvicirra, and Actinometra midtifida, will 

 show, however, that he has not written them out according to his own system, for none of 

 them have any brackets at all, although in each case he knows of individuals in which 

 the number of arms is not an exact multiple of ten. 



There is another point too which he does not seem to have fully considered in the 

 construction of his formulae. The multibrachiate Comatuke, such as Antedon occulta 

 (PL XLVIII. fig. 1) and Actinometra stelligera (PL LVIII. fig. 1), in which the successive 

 arm-divisions typically consist of two joints each, the axillary without a syzygy, are as a 

 rule extremely regular in their characters. But the case is quite different in those forms 

 which typically have three distichals and three palmars with syzygies in both the axillaries. 

 It is extremely rare to meet with examples of these species in which one or more of the 

 three-jointed distichal and palmar series are not replaced by two jointed series without 

 syzygies in the axillaries. Thus, for instance, I have described specimens of Actinometra 

 parvicirra with eighteen and twenty arms respectively, in which half the distichal series 

 were two-jointed, and the other half three-jointed ; and a similar irregularity occurs 

 among the palmars. In twelve individuals, however, ninety-six out of one hundred and 

 eleven distichal series, and sixty-seven out of seventy-six palmar series, were three-jointed; 2 

 and I was thus definitely enabled to make out the characters of the type and to write its 



i "Alert" Report, p. 155. 2 Trans. Linn. Soc. Land. (Zool.), 1879, ser. 2, vol. ii. pp. 44, 45. 



