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



6. The Valida-grorxp. 



Two articulated distichals ; the first arm-syzygy in the third brachial. 



Remarks. — Two somewhat different types of structure are comprised in this group. 

 viz., forms with two palmars like the distichals, and forms with three palmare of which 

 the axdlary is a syzygy. In addition to these there are also the species, like Actinometra 

 elongata and Actinometra simplex, which have normally no palmar series at all (PI. L VII. 

 fig. 2 ; PI. LIX. fig. 1). With one exception, which is in the National Collection, these 

 are the only species of the genus which have such a simple ray-structure ; and I do not 

 know of any other form which has its subsequent arm-divisions of the same character as 

 the distichals. This is in remarkable contrast to the number of Antedon-species which 

 have the same general formula and belong to the Spinifera- and Palmata-gvoups. 



On the other hand a few Actinometra-sipecies like Actinometra rotalaria and Actino- 

 metra valida have one or more arm-divisions beyond the distichal axillary, each consisting 

 of three joints with the axillary a syzygy, an arrangement which does not occur in 

 Antedon. Some of the individuals which have been distributed by the Godeffroy Museum, 

 under Liitken's MS. names Actinometra intricate!, and Actinometra trachygaster, are of 

 this character ; but other specimens bearing the same names are tridistichate, and therefore 

 resemble Actinometra parvicirra (PI. LXI. figs. 1, 5). The two types are so intimately 

 connected, however, that it is impossible to consider them as representing separate 

 groups. Thus, for example, I have described examples of Actinometra parvicirra in 

 which half the distichal series consisted of two, and the other half of three joints ; and a 

 specimen in the Vienna Museum presents a similar variation. Then again, two (or more) 

 three-jointed distichal series occur in the unique specimens of Actinometra elongata and 

 Actinometra valida, and in the figured one of Actinometra rotalaria (PL LVII. fig. 2 ; 

 PI. LIX. figs. 2, 3) ; while in the two last two-jointed palmar series may also present 

 themselves as a variation on the normal three-jointed type. 



Under these circumstances it is clear that these variations in structure are not 

 morphologically equivalent to the changes in the position of the arm-syzygies which 

 characterise the Stelligera- and Fimbriata- groups (PI. LVIII. fig. 1 ; PI. LXII. fig. 3), the 

 former having a syzygy betiveen the first two brachials, while the latter has a syzygy in 

 the second brachial ; and until the discovery of hitherto unknown species renders the 

 number of forms comprised in the P~a^'<ia-group much more considerable than it is at 

 present, we shall do best to include in it all those bidistichate species which have the first 

 arm-syzygy in the third brachial, whether the palmar series consists of two or of three 

 joints. 



All the members of the group, as at present constituted, are confined exclusively to 

 the Eastern Archipelago, including the Fiji and the Friendly Islands. 



