362 THE VOYAGE OF H.M.S. CHALLENGER 



antarctica. Although the two species are scarcely distinguishable when viewed on the 

 actinal surface, the difference between them in their abactinal aspect is most striking. 



I felt at first sight disposed to rank this form as only a variety of Porania antarctica, 

 but from the fact that the examples of Porania glaber are of large size and all identical 

 in character, also that a large series of specimens of Porania antarctica were collected 

 ranging from those very young up to those equal in size to the examples of Porania 

 glaber, and that all these maintain their characters without exception, I am inclined to 

 consider the differences above noted as worthy of specific recognition. 



4. Porania spiculata, n. sp. (PL LIX. fig. 4). 



This form is very nearly allied to Porania glaber, with which it agrees in all the 

 general points of structure. The chief difference is that in the present species the whole of 

 the abactinal membrane is furnished with small, widely spaced, spicule-like spinelets, which 

 are buried in the membrane, and are only to be seen with the aid of a magnifying-glass. 



None of the examples attain the size of the adult Porania glaber. It is also to 

 be noticed that the disk is large, and that the rays appear to proceed more abruptly from 

 it ; the abactinal surface is less convex, and incapable of the great inflation seen in 

 Porania glaber ; the marginal spines are relatively smaller, and have a tendency to be 

 directed downward rather than horizontally ; the outer series of spines on the adam- 

 bulacral plates are robust and chisel-shaped, and do not appear to be normally more than 

 one-third longer than the inner series, often not so much. On the median and outer part 

 of the furrow supplemental outer spines are very frequently present, and being situated 

 obliquely in relation to the normal outer spine, which they resemble in all respects, the 

 appearance is produced of an oblique series of three spinelets on a plate. On the outer 

 part of the ray this occurs on nearly every plate, and is probably normal. 



Although these differences are not very striking superficially, I have been led to con- 

 sider them worthy of specific recognition, after a careful study of the series of specimens 

 belonging to this form together with those of Porania glaber and Porania antarctica. 

 If the correctness of this view be not admitted, Porania glaber and Porania spiculata 

 would have to be ranked as varieties of Porania antarctica, a course which in my opinion 

 would demand an amount of elasticity within the scope of a species scarcely justifiable in 

 our present knowledge of this genus. 



I can detect no differences worthy of note between the example from the Arrou Islands 

 and those from off Heard Island. One would almost feel disposed to entertain.some doubt as 

 to the accuracy of the label of the specimen from the Arrou Islands, but I have no reason to 

 give for this suspicion excepting the presence of the species under notice in such a locality. 



The colour in alcohol is a brownish grey, with traces of a dark purple tint on **je 

 largest examples, which was probably the normal colour during life. 



