ASTEROIDEA 147 



aurorae the podia force apart the margins of the furrow. In large examples at the 

 middle of furrow 3, or rarely 4, podia can be counted in a transverse series. In the 

 example oi ftiscus there are 84 podia to a ray while in a slightly smaller example of 

 aurorae there are 108. 



The dorsal aspect of the 2 species is practically the same. Both are covered with 

 close-set blunt papillae. But the structure of the spinelets is different, those of aurorae 

 being almost brush-like from a multitude of sharp thornlets which do not so obviously 

 form the margins of radiating lateral flanges. In contrast to sladeni and georgianus the 

 pedicel of the paxillae is low, as in ftiscus and antarcticiis. 



In aurorae both series of marginal plates are clearly discernible. In large specimens 

 they become separated in the interbrachial arc by several series of smaller intermarginals 

 similar to the lateral abactinals. The marginals carry 8 or 9 spinules about as large as 

 adjacent actinal spinules. 



The largest of the Discovery specimens, from St. 370, measures R 1 12 mm., r 44 mm., 

 br48mm. The smallest specimen, from St. 371, measures R 34 mm., r 12 mm., br 

 14 mm. The others are scattered between these limits. 



The structure of the abactinal spinelets described and figured by Koehler (1920, pi. 

 75, fig. i) is highly characteristic. Each spinelet is clavate. The enlarged tip bristles 

 with numerous slender thorns buttressed to their neighbours by trabeculae which in- 

 crease in number as the thorns or spicules lengthen. Separating the trabeculae are holes 

 of varying size (Figs. C, 2, 20). When the spinelets begin to degenerate as the animal 

 reaches fully aduh size, the trabeculae are absorbed first, leaving a row of serrations 

 along the adjacent borders of neighbouring spicules (fig. 2a). In some large specimens 

 the spinelets are not so well developed as shown by fig. 2 (St. 366) but may have the 

 shorter spicules of fig. i (St. 371) which represents a smaller specimen (R 40-50 mm.). 

 Koehler (1920, pi. 75, fig. i) gives photomicrographs of spinelets but the figures have 

 suffered by the routing of the background. Koehler's fig. i c is nearest to the spinelets 

 of the Discovery specimens. The pedicels of the abactinal paxillae are usually a trifle 

 higher than those of typical antarcticus (figs, i b, 3^). 



The adambulacral furrow spine, and even the second spine, is usually strongly com- 

 pressed or spatulate as is clearly shown by Koehler's figures. 



A specimen with R 43 mm. already has the gonads well developed. 

 Note on specimen from St. MS 71 (Fig. C, 3-3^). This specimen from South 

 Georgia is not at all typical but it is nearer to aurorae than any other species. It measures 

 R 48 mm., r 16 mm., R=3r, br 18 mm. The gonads are well developed. The spinelets 

 have rather fewer and coarser thornlets than has typical aurorae while the pedicel of the 

 paxillae is higher than in typical southern specimens (compare figs. \b and ■^d). The 

 spinulation of the abactinal surface passes into that of the actinal without a con- 

 spicuous break, although small inferomarginals with 5-8 spinelets can be recognized in 

 contrast to the actinals which usually carry 3, but adjacent to furrow, 4 or 5. Most of 

 these groups of actinal spinelets are modified into pedicellariae, the 2-5 spinelets being 

 bent toward a common centre (fig. 3^). Although such spiniform pedicellariae are 



