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Show 736 MR. P. H. CARPENTER ON THE [Dec. 19, division of 3 joints, the axillary with a syzygy. Subsequent divisions of two joints united by syzygy. ^ ^ nova-guinea. 1(2)A'RDP^' typica. lA'RD-. B. Second and third radials united by ligament. Many arms. First ray-division of three joints, the axillary with a syzygy. a. Rays divide three times. Subsequent divisions like the first. robustipinna. A'(D)P-l. japonica. V3A'R?« parvicirra. 3ADP-. |8. Rays may divide five times or more. I. Third and fifth ray-divisions like the first. Second and fourth divisions of two joints, the axillary without a syzygy. altemans. 3A'RPP"^. II. All ray-divisions like the first. schlegeli. 3A'RDPf. bennetti. 3A'RDP£. peroni. 3A'RDP^. Prof. Bell's formulas do not give any thing like a proper idea of the characters of Act. nova-guinea and Act. typica, especially the latter. Both species are among "those rare cases in which divisions extend beyond the palmars;" and Prof. Bell should therefore have made use of his symbols P' and P". These two would have sufficed for A. nova-guinea, which has only two axillaries beyond the palmars. Strictly speaking, however, neither P, P', nor P" have any proper place in the formula; for the palmar and subsequent axillaries are not syzygial joints homologous with the distichal axillaries, any more than the radial axillary is, either in these two species or in the Solaris group ; and as pointed out above, it is equally incorrect, for morphological reasons, to describe the first brachials as being syzygial joints homologous with the third brachials 1 It is absurd to put the D within brackets in this formula, because the only specimen described has no syzygy in the axillaries of three out of the nine primary arms. I have described a specimen of Act. parvicirra in which five out of the ten distichal axillaries have no syzygy, and another in which there are four axillaries with and four without a syzygy. Here therefore we have a character which " frequently though not always obtains" just as in Act. robustipinna. W h y is the one case noted in the formula but not the other ? Prof. Bell's experience of the variations in these characters must surely have taught him that it is the exception and not the rule for all the distichal and palmar series of any many-armed specimen to be exactly alike, and that a specific diagnosis must be based on the characters of the majority. When, however, some •specimens of any type have distichals or palmars, and others may be altogether without them it is useful to put the D or P within brackets ; and this shoidd have been done in Bell's formula for Act. parvicirra, as I shall shortly point out. |