HBXACTINELLIDA. 5 



divide into two or three terminals tipped with toothed discs. Microdiscohexasters 

 (IV. Ig), 4:3fi in diameter, have slender primary rays, each 6 • 5;a in length, ending in a 

 plano-convex capitulum,f from the distal convex surface of which are given off disc- 

 tipped terminal rays of two lengths. 



This species is placed under Hyalascus on account of the absence of calycocomes, 

 this negative character being the chief one which separates the genus from Rossella. 



The three other known species of Hyalascus (5, p. 87), viz., 11. sagamiensis Ij., // 

 similis Ij., and H. giganteus Ij., are Japanese. They are all vase- or sack-shaped ; the 

 first-named species is vase-shaped, has an orifice with everted lips, and is much 

 contracted below ; the second, which Ijima regards as possibly identical with the first, 

 has a basal stalk ; the third is in the form of a large flattened sac with a plain orifice. 

 In all the Japanese species the autodermalia are mainly pentactins, but in the 

 Antarctic species these spicules are mainly hexactins, pentactins being very rare. 



Near the lower end of the sponge is a small conical elevation about 1 • 3 cm. in 

 height, with a central axis of diactins. The autodermalia have here undergone a 

 remarkable change (IV. Ic-c^) ; they have become more or less fused together, and 

 have lost one or more of their rays, while the spines resemble the flat articular surfaces 

 and tubercles on the desmas of Lithistid Sponges. All the stages of transition can be 

 traced from a slightly modified pentactin to a long desma-like form, such as that figured 

 in Plate IV., fig. Icl Probably these changes have resulted from irritation set up by 

 contact with some foreign body, just as basidictyonalia form at the point of attachment 

 of many of the Hexactinellida. On another part of the dermal surface is a small round 

 patch, 1 ■ 5 cm. in diameter, lighter in colour than the rest of the surface, occupied by 

 small densely crowded autodermalia. Ijima (5, p. 90) refers to similar patches on the 

 gastral surface of //. sagamiensis, " due to excessive local accumulations of gastralia." 



Dredged from 914 metres (500 fathoms) off Mount Erebus. 



Rossella Antarctica. 

 (Plate I., figs. 1 to 4, and Plate IV., figs. 2a to g, and 3a to k.) 



1872 Rossella antarctica, Carter (1, p. 409, pi. xxi.). 

 187.5 „ „ (2, p. 114, pi. X., fig. 4). 



1887 „ r. E. Schulze (6, p. 139, pi. Iv.). 



Three specimens of this species were obtained. They were brought up in the 

 same haul from Winter Quarters, No. 10 hole, 130 fms. The specimens show a 



canals to the primary rays, and the spine-like nature of the secondary rays:— PI, IV., fig. 2d*-^ shows an 

 hexaotin-like monoxyhexaster with the axial canals extending but a very short distance from the centre, the rest 

 of each ray being solid. PI. VII., fig. Ih, shows a monodiscohexaster with the axial canals extending only to the 

 end of the basal thick portion of each ray. PL IV., fig. Sd" shows three spheroidal reduced hexasters in which 

 the primary rays have disappeared as rays, while the secondary rays remain as one or more spines, or even 

 spheres, attached to the central sphere, the axial canals being confined to the central node or sphere. PI. IV., 

 fig. 3d* (on the left edge of the plate) shows a monoxyhexaster in which one of the six secondary rays is becoming 

 reduced, this being a stage on the way to becoming a spheroidal reduced hexaster. 



t The descriptive term "capitulum" is suggested for the enlargement or swelling at the distal end of the 

 primary rays of calycocomes, aspidoplumicomes, strobilocomes, microdiscohexasters, etc. The shape of the 

 " capitulum " in these spicules calls to mind the receptaculum and capitulum of the CompositsB, 



2 H 2 



