114 THE VOYAGE OF H.M.S. CHALLENGER. 



distinct, cork-like cortex lying immediately below the dermal membrane, about 0"24 mm. 

 thick. Colour in spirit light brown. Texture corky, internally fibrous. Surface 

 fairly smooth. Dermal membrane fairly distinct and peeling off readily in strips. 

 Oscula apparently few, scattered, each on a small papilla, but the condition of the 

 specimen is such as to make it difficult to be certain of this. Pores (?). 



Skeleton. — Eadiately arranged. From about the centre of the base bands of 

 spiculo-fibre radiate to the surface, where they terminate in brushes of stylote 

 spicules with their points directed outwards and embedded in the dense cortex. 



Spicules. — (a) Megasclera ; of one kind only, viz., straight, smooth styli (PI. 

 XXIII. figs. 8, 8a), verging upon tylostyli, fairly gradually and sharply pointed, 

 often with the shaft slightly bulbously dilated at intervals ; size about 0"65 by 

 0*0126 mm. (b) Microsclera ; of one kind only, viz., isochelge, of very peculiar 

 form (PI. XXIII. figs. 86, 8c, 8(:/), with three distinct, rather palmate teeth at each 

 end and with a diamond-shaped " tubercle " (Carter) ; often the two anterior teeth 

 are seen to be connected together by their apices (PI. XXIII. fig. 8c); length about 

 0"044 mm. 



In addition to spicules the sponge contains a large amount of sand scattered 

 irregularly through it. Unfortunately there is only a single small specimen and that 

 not in very good condition. 



Locality. — Station 320, February 14, 1876 ; lat. 37° 17' S., long. 53° 52' W.; 

 ofi" the mouth of the Eio de la Plata ; depth, 600 fathoms ; bottom, green sand. One 

 specimen. 



Genus Sideroderma,^ Eidley and Dendy (Pis. VIII., IX.). 

 1886. Sideroderma, Eidley and Dendy, Ann. and Mag. Nat. Hist., ser. 5, vol. xviii. p. 348. 



Sponge massive, with mammiform processes on the upper surface, provided with a 

 more or less dense external rind, composed of tylote spicules horizontally arranged. 

 Megasclera, tylota ; microsclera, isochelse of various forms, and, at any rate usually, 

 sigmata and trichodras;mata. 



Only a single species of the genus, and of that only a single specimen, is present in 

 the collection ; but this difi"ers so markedly from all other known Desmacidonidse that it 

 seems advisable to found a new genus for its reception, more especially as we have been 

 enabled, through the kindness of Dr. R. v. Lendenfeld, to examine a second species 

 which occurs in his large collection of Australian sponges. The description of this 

 second species has not yet been published, but an examination of it has enabled us to 

 give a much more satisfactory generic diagnosis than would otherwise have been the 

 case. 



1 From Greek alinQo;, iron and 3ij,«a, skin. 



