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



has articulated distichals and palmars, but the first two brachials united by syzygy 

 (PI. LVIII. fig. 1), so that its formula is— a.2.2.^-. 



6. Whenever any arm-division, distichal, palmar, or any other consists of three 

 joints, the first two are articulated by ligaments, the second bearing a pinnule, and the 

 third (axillary) is a syzygy, just as in the first three brachials of Antedon eschrichti 

 (PL XXIV. fig. 11) and Actinometra meridionalis (PL LVI. fig. 1). When, however, 

 there are only two joints, and the second (axillary) is a syzygy, the first has a pinnule, 

 just as in the arm-bases of Actinometra fimbriata (PL LXII. fig. 3). 



Examples. — Antedon variipinna (PL XL VIII. fig. 5) and Antedon porrecta 

 (PL LII. fig. 3) ; Actinometra parvicirra (PL LXI. fig. 1) and Actinometra sentosa 

 (PL LXVI. fig. 4). 



7. The hypozygal of a syzygy is always united to the preceding joint by a muscular 

 articulation. 



The method of formulation which I have devised in accordance with the above rules 

 is as follows : — 



Like Professor Bell I use R to denote the syzygial union of the two outer radials ; 

 and I assume in accordance with Rules 3 to 5 that the first syzygy on the arm is in the 

 third brachial, unless otherwise stated. If it is in the second brachial I put 2br at the 



br 



end of the formula ; and if the first two brachials are united by syzygy -h- is used. 



In like manner, and in accordance with Rule 5, 2d and 2p would indicate that there are 



two distichals or two palmars, of which the axillary is a syzygy ; and -3- or 4v that the 

 two distichal or two palmar joints are themselves united by syzygy. 



The figures 1 or 2 alone would indicate that there is either only a single axillary 

 joint, or that the axillary is the second joint and bifascially united to its predecessor; and 

 a 3 would denote three joints of which the axillary is a syzygy. 1 If one figure occurs alone 

 in a formula it indicates the presence of distichals only ; two figures that palmars occur 

 as well ; and so on, an additional figure or letter (p\ p", p'") being added for each fresh 

 division, e.g., Actinometra alternans, 3, 2, 3, 2, Actinometra sentosa 3.2 (p. p'. br). 



This may be tabulated as follows : — 



Symbol used. 

 Character. Distichal. Palmar. 



One axillary joint, ..... 1 1 



Two joints united by syzygy, 



Two articulated joints, 



Two joints, the axillary a syzygy, 



Three joints, the axillary a syzygy, 



1 It would of course be more consistent to write 3d or 3p ; but the syzyj 



d p 



1 ~2 



2 2 



2d 2? 



3 



nature of the third (axillary) joint is such 



a constant character (Rule 6) that until an exception is met with, I prefer to use the figure alone for the sake of brevity. 



