592 COLLECTIONS PROM TIIE WESTERN INDIAN OCEAN. 



fibres, meeting the surface at various angles, and projecting slightly 

 above it, and of a system of crossing fibres connecting the two and 

 forming approximately rectangular meshes, their direction being 

 roughly vertical to one or other of the above systems. The mesh 

 is very variable in diameter, viz. from -07 to *24 millim., the former 

 chiefly at the surface. The diameter of the main fibre is , 028-"Q7 

 millim., not including the ditelous network of fibres of small dia- 

 meter which often surrounds the bases of the large primary fibres. 

 Colour of fibre pale amber-yellow ; no foreign bodies imbedded in 

 any part of the skeleton ; the fibre is homogeneous in appearance, 

 with the occasional exception of a faint granular axial line. 8ize 

 of sponge, 80-95 millim. (3-3| inches) in greatest diameter. 



Hah. African Island, Amirante group (gathered on beach). 



Distribution. "Indian Ocean?" (Pallas); Indian Ocean (Lamarck); 

 Mauritius {Hyatt). 



Hippospongia shraosa, Pallas, var. decidna, Hyatt. 



The other variety of the species is very distinct in its external 

 appearance, but on examination this is found to be due merely to 

 modifications of the same structural arrangements as those of var. 

 mauritiana. The surface is entirely broken up into small isolated 

 tufts, or short meandrine ridges, flattened externally, about 1-2 

 millim. in diameter (the ridges of mauritiana being 3 or 4 millim. 

 across), rising from a considerable depth, viz. 7-15 millim., and 

 commencing below by very narrow bases, and not expanding until 

 close to the surface. By the juxtaposition of these tall walls and 

 tufts, a number of freely intercommunicating, very narrow (2 to 2-5 

 millim. wide) and deep channels are formed, very different in appear- 

 ance from the subcylindrical and semi-tubular canals which repre- 

 sent them in var. mauritiana. The outward form of the sponge 

 is essentially similar to that of the other variety, but the speci- 

 mens are much larger ; the largest, an example of incrusting growth 

 about 30 millim. in average vertical thickness, measures 275 millim. 

 (11 inches) in greatest diameter; some smaller specimens attain 

 about twice the thickness. As in var. mauritiana, the tubular cha- 

 racter of the channels of the skeleton is much more strongly marked 

 on the lower surface, where (as observed by Hyatt) connecting 

 laminae of horny fibre frequently bridge over the spaces between the 

 summits of the tufts and ridges. The colour of well-preserved 

 skeletons is a rather pale amber-yellow ; those which have suffered 

 much washing on the beach are almost white. 



The general arrangement of the skeleton is similar to that of the 

 other variety ; but the following important differences are to be 

 noted :■ — (1) It is the primary and not the secondary fibres which 

 are the most distinct elements of the deep skeleton; they foim 

 continuous, almost straight lines, *4--5 millim. apart, and are 

 placed vertically to the surface. The primary fibres of the 

 outer surface form a decided pile of short projecting points, being 

 much more numerous than in var. mauritiana. (2) Owing to the 



