ACTINOMETRA TVPiCA. 197 



think that it can be regarded as generically distinct from 

 Actinomet7'a ; though there are certain features about it, 

 besides the stellate centrodorsal , which distinguish it in a 

 very marked manner from the other species of the genus. 



In ordinary Actiiwmetra-s^ecies ') the mouth is some little 

 way from the centre of the disc which is occupied by the 

 anal tube. But in Loven's specimens from Singapore , as 

 in Liitken's from Fiji and in others which I shall mention 

 immediately , the mouth , though not absolutely central , is 

 only slightly excentric , and the anus is at or near the 

 margin of the disc. In fact it is sometimes very difficult 

 to find the anus at all. In this respect the disc of Ac- 

 tinometra ttjpica approaches that of Antedon ; but the distrib- 

 ution of the ambulacra is not so symmetrical as in An- 

 tedon , and the anal interradius is always by far the largest. 

 The presence of a terminal comb on the oral pinnules , the 

 general features of the calyx , and the absence of sacculi 

 from the pinnule ambulacra also distinguish this type very 

 sharply from Antedon. 



Another character in which Act. typica differs from ordin- 

 ary Actinometra-?,^ec\.Q^ and also from Antedon , is in the 

 peculiar mode of union of the second and third radials 

 which 1 believe to be an imperfect form of syzygy , although 

 Loven ^) describes it as a ligamentous articulation. 



When two joints are united by syzygy, as the two outer 

 radials are in Act. juk^êsii, Act. Solaris, Act. robusta and 

 others, each of their apposed faces is marked by a num- 

 ber of low ridges , which diverge from the opening of the 

 central canal and extend towards the dorsal margin of each 

 joint, causing it to be delicately toothed ^). In the natural 

 positions of the apposed faces their ridges correspond 

 in position , so that the presence of the syzygy is indicat- 



1) Trans. Linn. Soc. Ser. 2. Zool. Vol. II. Fl. 1. PI. 2. figs 1, 8. 



2) Loc. cit. p. 228. 



3) This is well shown in fig. ID on Tl. XXXVI of Dr. Carpenter's meuioir 

 on Antedon rosacea. Phil. 'J'rans. ISBB. 



Notes from the Leydeii ÜMiiseuiii, Vol. Jll. 



