182 Proceedings of the Royal Physical Society. 



as does a third radial (E. 3). There are thus two third 

 brachial plates supported by the triangular second brachial, 

 and each of these two third brachials is, of course, the 

 starting-point for a long series, and thus the division of 

 the arm is brought about. It is interesting to note the be- 

 haviour of the syzygies under these circumstances. Normally 

 a syzygy occurs between the third and fourth brachial plates, 

 or, perhaps, more correctly speaking, in the middle of the 

 third brachial ; in this divided arm each of the two third 

 brachials has a syzygy (Fig. 2 Sg). The second brachial 

 plate supporting the two halves of the divided arm bears 

 no pinnule, as it normally does, but in each of the two halves 

 of the divided arm the first pinnule is given off on the out- 

 side, immediately above the syzygy {vide Fig. 2). 



The fourth and fifth arms of the right hand side arise in a 

 perfectly normal manner. On the left hand side the first, 

 second, and third arms arise as usual ; the fourth, however, 

 behaves just like the third arm of the right side — that is to 

 say, the second brachial acts like a third radial, and bears 

 two third brachials. The only difference concerns the 

 syzygies, and consists in the development of an extra syzygy 

 in the anterior half of the divided arm (Fig. 2, L. 4 a), in the 

 position shown in the figure {Sg.)} The fifth arm of the left 

 hand side arises as usual. 



The two extra arms are supplied with ambulacral grooves 

 by the corresponding bifurcation of the grooves belonging to 

 the divided arms. This is made sufficiently clear from the 

 figure (Fig. 1) without further description. It will be seen 

 from this figure that there is also a slight irregularity in the 

 mode of origin of the ambulacral grooves of the fourth and 

 fifth arms of the right hand side. In the inter-radial areas 

 the surface of the disk is rough and ^varty, especially near 

 the lips of the ambulacral grooves, and marked with more or 

 less concentric darker bands. 



I have made the whole description as short as possible, 

 relying rather on the figures to show the actual condition of 

 the specimen. 



1 On further examination it appears probable that the arm, R. 3 a, also 

 possessed an additional syzygy in the same position, at which the arm has, 

 unfortunately, been broken off. 



