354 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
Family AXINELLIDA. 
Sigmatotetraxonida in which the microscleres have usually been entirely 
lost by degeneration ; the megascleres are usually, in part or entirely, stylote ; 
the skeleton arrangement is usually, but not always, plumose ; and there are 
no spined echinating styli. 
HyYMENIACIDON CALCIFERA, n. sp. (PI. 38. fig. 19; Text-fig. 24.) 
There are two specimens cf this new species in the collection, each of 
them growing over a mass of calcareous Lamellibranch shells and coral. 
The sponge forms a rather thin sheet over the coral and shells and is easily 
stripped off. The largest specimen measures 50 mm. by 50 mm. The film 
of sponge varies from 1 to 6 or 8 mm. thick. 
The surface (PI. 38. fig. 19) is irregular, and covered with slight pro- 
minences and depressions, and also exhibits in parts a coarse and very 
irregular reticulate appearance, due to the skeleton arrangement. An 
examination of the photograph will, however, convey a better impression of 
the appearance of the surface than a written description possibly can. 
The oscula are fairly numerous, usually rather small, and scattered 
irregularly over the surface. Here and there large oscula occur measuring 
3mm. to 5 mm. in diameter, but the average size of the oscula does not 
exceed 1 mm. 
The pores are very numerous and are not confined to special pore-areas. 
Tn some parts of the sponge no pores could be seen, but in others they were 
very plentiful, especially in those parts of the specimen which had a reticulate 
appearance. They are very small, and do not exceed 0-1 mm. in diameter. 
The colour of the sponge in spirit was a clear chalk-white. 
The texture is rather lax, and the film of sponge when stripped from its 
support is limp and easily injured. 
Skeleton arrangement. 
The main skeleton consists of an irregular reticulation of spicule-fibres, 
which are quite small and scattered in most parts of the sponge, but 
which here and there in the sponge are much larger and form a 
definite, but irregular reticulation. In this case the spicules composing the 
fibre may be as many as 10- to 15-serial, but usually they do not lie more 
than 5-serial in the fibre. At the surface these fibres usually project very 
slightly. 
The most curious and characteristic feature of the sponge lies in the fact 
that there is a distinct dermal skeleton formed of calcareous aster-like 
spicules. These forma dense and regular covering over almost the whole 
surface of the sponge, and apparently never le more than one thick. It 
does not seem reasonable to suppose that these spicules are formed by the 
