TETRAXONIDA 285 



With regard to the types of Desmacidon setifer, D. spmigera, Homoeodictya trigona, 

 H. verrucosa and H. obliqiddens we may say that in external appearance there is Hterally 

 nothing to choose between them. In the arrangement of the spicules in the main skeleton 

 there is the same similarity, and in the shape of the microscleres there are the same 

 peculiarities. It is only in the dimensions of the spicules, and more particularly of the 

 megascleres, that distinctions may be made between them. The dimensions of the 

 spicules are as follows : 



Oxea (mm.) Chelae (mm.) 



D. setifer o-88o-i-o o-023-o-03 



D. spmigera 0'73i 0-024 



H. trigona 0-740 0-060-0-07 



H. verrucosa 0-52 0-027-0-037 



H. obliquidens 0-528-0-672 0-038-0-055 



The variation in the dimensions of the oxea in these five forms is from 0-52 to 

 i-o mm., and in the chelae from 0-023 to 0-07 mm. This, on the face of it, is quite a con- 

 siderable range. On the other hand, the five forms afford a convenient series of inter- 

 mediates from one extreme of measurement to the other, both in the oxea and in the 

 chelae, and it is obvious that if a large collection of specimens were examined and an 

 attempt made to divide them into species on the basis of spicule measurement, the 

 attempt would prove to be impossible. This is actually what has happened in the case 

 of the Discovery specimens, in which the average measurements of the spicules range as 

 follows: oxea o-35-o-7 by o-oi-o-025 mm., and chelae o-028-o-o6 mm. chord. 



From direct observation and comparison of the 32 specimens of the present collection, 

 I have no doubt whatsoever that they are all conspecific : and since the range of variation 

 in the dimensions of their spicules is the same, or practically the same, as that found 

 in the five species tabulated above, especially in view of the strong resemblance these 

 five species have to each other in all other respects, I can see no alternative to regarding 

 them as synonymous, even though it means the admission that the oxea in this species 

 may have the surprisingly wide range of dimensions shown, from 0-35 to i-o mm., and 

 that the chelae may vary from 0-023 ^^ °'°7 ™^^- 



Distribution. Wilhelm Land ; Victoria Land ; Graham Land ; Falkland Islands. 



Isodictya delicata (Thiele). 



Desmacidon delicata, Thiele, 1905, p. 435, figs, i, 55. 

 Occurrence. St. WS 109: Falkland Islands, 145 m. 

 Distribution. Admiralty Sound. 



Isodictya antarctica (Kirkpatrick) (Plate LI, fig. 4). 



Desmacidon kerguelensis , var. antarctica, Kirkpatrick, 1908, p. 37, pi. xix, fig. i, pi. xxiii, fig. i. 



Occurrence. St. 42: South Georgia, 120-20401.; St. 155: South Georgia, 260 m.; St WS 73 : 

 Falkland Islands, 121 m.; St. WS 83 : Falkland Islands, 137-129 m. 



Distribution. Victoria Land. 



