i82 DISCOVERY REPORTS 



The largest specimens measure as follows : St. i8i, R 142 mm., r 42 mm., br 45 mm. ; 

 St. 148 (South Georgia), R 160 mm., r 46 mm., br 50 mm. 



Koehler has fully described and figured the external features of this species, including 

 the characteristic spinelets. His type specimen from Adelaide Island and Alexander I 

 Land were all small, none exceeding R 28 mm. However, the Australasian Antarctic 

 Expedition secured material from off Adelie Land, the largest specimen of which has 

 R 120 mm. 



St. 599 is very near the type locality and the largest specimen has R 74 mm., r 21 mm., 

 br 22 mm., and conforms well to Koehler's description. A curious feature of this 

 specimen, as well as others from different localities, is the position of the subambulacral 

 comb which consists of 3-5 basally webbed spines. The normal position of the comb is 

 probably a slightly curved, obliquely transverse, line from the outer adoral to the inner 

 aboral corner of the plate. However, this line shifts so that the slightly curved comb 

 (convex side adoral) is almost at a right angle to furrow; or the axis of the comb may shift 

 so that the comb becomes more nearly parallel with the furrow series and closer to it. 

 This last posture is rather characteristic of the specimens from Schollaert Channel, 

 Palmer Archipelago, although oblique transverse combs are found — especially on the 

 outer part of ray. In contrast the specimens from other localities such as Adelaide 

 Island, South Sandwich Islands, South Georgia, while showing the extremes of varia- 

 tion have the majority of the subambulacral combs obliquely cross-wise on the plate. 



A variable feature of the species is the size of the actinal interradial area which is 

 associated with size of the disk. The type form has the smallest disk (Koehler, 191 2, pi. 

 3, fig. 5, young; 1920, pi. 32, fig. 9, adult). The disk may become much broader, and the 

 interbrachia more rounded than in Koehler's pi. 33, fig. 4 (1920), with correspondingly 

 enlarged actinal interradial areas. This holds true for all the South Georgia specimens, 

 the large specimen from St. 181, that from St. 182, but not for the example from St. 371 

 (R 122 mm.). It is difficult to express the difference in terms of actinal plates, as they are 

 of about the same number in large specimens but differ in spacing. Eight to ten paxillae 

 may be counted along the interradial line from mouth-plates to margin. The series 

 parallel to furrow is not always very regular, outside the first, which reaches to within a 

 short distance of tip of ray. In the triangular actinal interradial area there are usually 

 about 6 chevrons of paxillae, with a few irregular additional paxillae next to the first 

 inferomarginals. 



The abactinal skeleton is a close mesh, fairly regularly 4-sided on the lateral parts of 

 the area but irregular along a broad radial band. The lateral abactinal plates (the bases of 

 the paxillae) have 4 rather long lobes which touch or overlap 4 neighbouring plates. In 

 the radial area the plates are irregular and are frequently 3- to 5-lobed, with short 

 connecting ossicles in old specimens. The papulae are found all over the ray and disk and 

 there are commonly 2-5 to an area in large specimens, but only i to an area in small 

 examples (and on outer third of ray of large specimens). 



The interbrachial calcified pillar is slender and flexible, and expands slightly where it 

 joins the abactinal skeleton. The membranous septum in which the pillar lies expands 



