PAKT 5 A MONOGRAPH OF THE EXISTING CREN'OIDS 425 



Heliometra glacialis rather than its fellow, P 3 . In the middle and outer pinnules there 

 is a good deal of variation in the extent of modification in the two basal segments, 

 but they are never so much flattened and so nearly trapezoidal in form as they are in 

 the larger Heliometra glacialis. 



The axial opening on the ventral surface of the centrodorsal is rclativelv much 

 larger in Solanometra antarctica than in Heliometra glacialis so that it reveals a large 

 number of the internal ridges which separate the inner openings of the cirrus canals. 

 Five of these, interradial in position, are much more prominent than the rest. The 

 basal grooves on the ventral surface of the centrodorsal are scarcely more distinct than 

 they are in Heliometra glacialis, but on the other hand the dorsal surface of the radial 

 pentagon bears a very well-defined basal star, of which there is rarely any trace in 

 that species. 



The rosette lies deeper in Solanometra antarctica than it does in Heliometra gla- 

 cialis, and the basal rays connected with it are unusually stout. This is most notice- 

 able when they are isolated. In some cases then- distal ends appear on the exterior of 

 the calyx. 



In Solanometra antarctica the slope of the articular faces of the radials (see Part 2, 

 plate 4, figures 997, 998) is more uniform than it is in Heliometra glacialis, and there 

 is much less of an angle between the upper and lower parts of each face. As a conse- 

 quence of this, more of the large muscular fossae is visible when the calyx is viewed 

 ventrally. The ridges which separate the muscular fossae from the interarticular liga- 

 ment fossae are much more nearly horizontal than is the case in Heliometra glacialis, 

 so that the two pairs of fossae are of very unequal size. 



Carpenter described Antedon australis as follows. 



The centrodorsal is hemispherical, thickly covered with about L cirri. The cirri 

 have 25-30 segments, nearly all of which are longer than broad. The later segments 

 are laterally compressed, and their dorsal edges project considerably beyond the bases 

 of their successors, thus giving rise to a strong spine on the last few segments. The 

 young cirri around the dorsal pole resemble the mature form, but have fewer segments. 

 The marginal cirri may have 30 smooth and elongated segments which only develop 

 spines quite late. 



The radials are just visible. The IBr! are short and nearly oblong, but little 

 incised for the axillaries, which are broadly pentagonal or triangular, with a slight 

 backward projection in the middle of the base. 



The 10 arms are about 60 mm. long. The brachials are somewhat overlapping, 

 but the proximal brachials are not tubercular. After the second syzygy the brachials 

 are shortly triangular, gradually becoming quadrate, but always much broader than 

 long. 



Syzygies occur between brachials 3+4, 9 + 10 and 14 + 15, and distally at intervals 

 of 3 or 4 muscular articulations. 



P! is flagellate, about 12 mm. long, and is composed of 45 short segments, of which 

 the basal are broad, flattened, and somewhat carinate. P 2 is sometimes nearly equal 

 to, and sometimes shorter than, P,. P 3 is also shorter, with stouter segments, most of 

 which are distinctly longer than broad, and generally bears a fusiform genital gland. 

 The following pinnules are more massive with squarer segments, which become elon- 

 gated further out, while the two basal ones are flattened and trapezoidal with their 

 apposed edges incurved. 



