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



3. Hexaiolagia collaris, n. sp. 



Spines in opposite pairs in three diameters, crossed in the common centre, straight, cylindrical, 

 pinnate, with three to fonr pairs of opposite lateral branches or regular pinnulse. All six spines are 

 equidistant, of equal size and similar form, and lie with their branches nearly in one horizontal 

 plane. Therefore they are similar to the six radial rods in the coUar septum of many 

 Spyroidea and Cyrtoidea {e.g., Calpophcena, PI. 53, fig. 18). 



Dimensions. — Length of aU six spines 018, of their basal pinnule 0-04. 



Habitat. — East of New Zealand, Station 169, surface. 



4. Hexaplagia australis, n. sp. 



Spines unequal, at unequal intervals, not opposite in pairs, cylindrical, irregularly branched and 

 curved. In this species a definite arrangement of the six different spines could not be detected. 

 Dimensions. — Length of the spines 0-2 to O'S, of their largest branches 01. 

 Habitat. — South of Australia, Station 160, surface. 



Genus 391. Plagonium,^ Haeckel, 1881, Prodromus, p. 423, 



Definition. — P lagonida with six radial spines, arising in two opposite divergent 

 groups from the two poles of a common central rod. 



The genus Plagonimn differs from the preceding closely allied Hexaplagia in the 

 remarkable peculiarity, that the six radial spines do not arise from one common central 

 point, but from the two poles of a common horizontal middle rod ; three divergent 

 spines on each pole. The skeleton of Plagonium exhibits therefore the same remark- 

 able form which is found in the isolated spicula of numerous Beloidea (e.g., 

 Lampoxantliium lyunctatum, Spharozoum p>unctatum), and bears the same relation to 

 Hexaplagia that Plagonidium does to Tetraplagia. 



1. Plagonium sphcerozoum, n. sp. (Pi. 91, fig. 6). 



Spines straight and stout, three-sided prismatic, about twice as long as the common middle rod, 

 irregularly branched or nearly vertieiUate, with short thorny branches. Similar to the single 

 spicula of some species of Sphcerozoum, and Lampoxanthium. 



Dimensions. — Length of the spines 012, of the middle rod 0"06. 



Habitat. — Equatorial Atlantic, Station 347, surface. 



* Pfo^oniiim = Side-article ; wAtiyof, umm. 



