246 PERCY SLADEN TRUST EXPEDITION 



from one or more of the original types (there were several specimens on the same mass) 

 seems also certain, for one of the slides is labelled " Bk. Coll. 701. Mauritius," and this 

 number and locality are quoted in the original description. Moreover the size and form 

 of the large spherasters are extremely characteristic (vide infrd, p. 269 and PL 48, fig. 8). 



The genus Magog therefore must fall to the ground, (l) as being based upon a serious 

 misconception as to the characters of the type species, which is a genuine Chondrilla, 

 and (2) because even were the type species as represented it would have to be regarded 

 merely as a reduced Aurora. 



It seems quite probable that the genus Chondrilla itself has been derived from 

 Aurora by complete loss of all the megascleres, and not only of the triaenes. 



10. Aurora j^rovidentice n. sp. 

 (PL 46, fig. 2.) 



This species is represented by a thin crust, about 25 mm. in greatest breadth, which 

 has apparently been pared off some other object, or possibly sliced otf from the surface of 

 a larger specimen of the same species, but if the latter hypothesis be correct it is remark- 

 able that the specimen in question is missing from the collection*. The shape of the 

 fragment is quite irregular. The upper surface is fairly smooth and shows two sieve-like 

 groups of openings, of very varying size, which are evidently oscula. There are a large 

 number of small calcareous foreign bodies scattered over the surface. Colour in spirit 

 pale, dull yellow. Texture cavernous. 



There is a thin, fibrous cortex, about O'l mm. thick, containing numerous large and 

 small spherasters in its outer portion, and pierced at wide intervals by inhalant chones. 

 Each chone is divided into ectochone and endochone by a thin diaphragm perforated by 

 a pore. The ectochone terminates externally in a single (?) dermal pore, the endochone 

 merges indistinguishably into a subcortical crypt. 



In the deeper part of the sponge the skeleton consists of loose bundles of oxea 

 running towards the surface, with many loose oxea scattered between. As these bundles 

 approach the surface they spread out into brushes composed mainly of slender oxea and 

 orthotrisenes, the cladi of the latter being extended in or beneath the fibrous cortex, 

 below the layer of lai^ge spherasters. 



Spicides : — (1) Orthotrisenes (Plate 46, fig. 2 a). Shaft varying from gradually and 

 finely pointed to rounded or even knobbed at the extremity. Cladi usually simple, 

 gradually and sharply pointed ; rarely bifid for a short distance at the extremity. Shaft 

 measuring about O'o by 0*017 mm., but variable ; cladi about 0'14 by 0"013 mm., but also 

 variable. 



(2) Anatrisenes (figs. 2 6, 2 h', 2 h"). Minute, with long, hair-like shaft and widely 

 extended cladi curving backwards almost on arcs of a sphere. Shaft about 0'37 by 

 0"001 mm. Cladome about 0"0164 mm. across from tip to tip of cladi; cladi about 



* The fact that Aurora [hops) membranacea [Hentschel 1909] forms a thin crust suggests that A. pro- 

 videntice not improbably has a similar form. 



