A MONOGRAPH OF THE EXISTING CRINOIDS 263 



again. Beyond this point, however, there is much variation. The pinnules of the 

 next three or four brachials are considerably longer and stouter than P ( , being the 

 largest pinnules on the arm. In those arms that spring directly from the IBr axillary, 

 so that there is no P D , the largest pinnules are usually P 3 and P c . When, however, a 

 IIIBr axillary is present, the arm borne on its inner face usually has its largest pin- 

 nules on the sixth and seventh brachials, while on the outer arm they are usually on 

 the fifth and sixth (P 2 and P b ).^But this arrangement is very far from being a con- 

 stant one. The next two pinnules after the large pair may also be of considerable 

 size and composed of somewhat elongated segments, but in other cases they show a 

 considerable alteration both in the size and in the character of their component 

 segments. 



Carpenter noted that the double row of lateral projections on the segments of 

 these proximal pinnules is developed in rather a singular manner. Their basal seg- 

 ments are somewhat flattened against the arm and the upper edge of their broad 

 dorsal surface is sharpened and more or less carinate, while its distal end is marked 

 by a median process of variable prominence. As the following segments lose their 

 flattened appearance and become more rounded the carination of the upper edge 

 develops into a strong blunt process at the distal end of the segment on its inner side, 

 while the mediodorsal prominence passes into a corresponding process on the outer 

 side. There is much variation, however, in the exact nature and mode of development 

 of these processes. 



It was remarked by Carpenter that the frequency of the division of the post- 

 radial series in this species, and therefore the number of the arms, is subject to great 

 fluctuations. He said that IIIBr series occur only in the single specimen, which he 

 named Antedon crenulata, and sometimes also in the form described by Bell as Antedon 

 irregularis. A large number of individuals were obtained by the Alert, and the major- 

 ity of them have two or more IIIBr series, though in others, as in the Challenger 

 specimen, IIIBr series are entirely absent. Bell gave the number of arms as ranging 

 from 11 to 22, but Carpenter said he seemed to have overlooked one example in which 

 there are 25. The occurrence of an individual with only 11 arms makes it quite 

 possible that a 10-armed variety of this protean type may eventually be discovered. 

 In fact, according to Carpenter, the two individuals that he formerly called Antedon 

 dubia are not improbably of this nature. One of them has two IIBr series, and the 

 other only one. But in each case they result from regeneration of the arm at the 

 syzygy between brachials 3 + 4. This may perhaps have originally supported a 

 IIBr axillary, or it may have given rise to only one arm, which was replaced by two 

 after fracture as it so often the case. Under these circumstances Carpenter thought 

 it safer to assign this species a place among the 10-armed species to which it can defi- 

 nitely be referred if ever an individual is found in which IIBr series are entirely 

 absent. He said that no harm will be done if it never turns up, and should it do so 

 it will run less risk of being baptised as a new species. 



The specimens referred to by Carpenter as Antedon dubia are simply young 

 individuals undergoing adolescent autotomy. But Carpenter was not aware of this 

 curious and wasteful method of arm reduplication. 



Carpenter said that there is one point relating to the extremely variable char- 

 acters of this species that seemed to him to be of special importance. The variations 



