SPONGES. 19 



This important genus is represented in the present collection by only two species, 

 but both of these are of exceptional interest owing to their unusual habit, the one 

 being a typical sand-sponge and the other a very thinly encrusting form. 



Myxilla arenaria, n. sp. 



Sponge massive, irregular ; intensely and coarsely sandy, with a distinct translucent 

 dermal membrane visible in places. Pores and vents (?). Greatest diameter of 

 type (R.N. 263) 30 millims. Colour (in spirit) light brown (the colour of the sand) 

 or grey ; texture very friable. 



Main skeleton composed almost entirely of sand, with slender strongyla scattered 

 between the sand-grains and also collected in loose, wispy fibres ; and with small 

 spined tylostyli or styli very sparsely echinating the sand-grains. Dermal skeleton 

 composed of slender strongyla scattered irregularly in the dermal membrane. 



Spicules. (1.) Strongyla, perhaps sometimes faintly tylote ; straight, slender, 

 smooth ; measuring about 0'146 millim. by 0'002 millim. 



(2.) Spined tylostyli or styli ; straight ; gradually and very sharply pointed at the 

 apex ; rather sparsely and irregularly spined, chiefly at and near the base, which is 

 generally distinctly tylote. Size variable, say about O'OSG millim. by 0'004 millim. 

 (near the head, including spines). 



(3.) Tridentate isochelse, with small teeth and fairly stout, strongly curved shaft ; 

 length about 0'014 millim. ; not very abundant. 



(4.) Slender sigmata ; C-shaped and contort ; measuring about 0'032 millim. from 

 bend to bend, but rather variable in size. 



As usual in sand-sponges, the proper skeleton is reduced in accordance with the 

 arenaceous habit. In external appearance the sponge closely resembles PwrespereUa 

 bidentata, and might easily be mistaken for that species. 



RN. 263 (Gulf of Manaar) ; 266 (pearl banks off Aripu). 



Myxilla tenuissima, n. sp. Plate XL, fig. 5. 



Sponge extremely thin, encrusting. The single specimen forms a small crust of a 

 pale yellowish colour growing on a mass of calcareous worm-tubes in association with 

 Discodermia emarginata. Vents and pores not seen. 



The skeleton consists partly of slender tylota, scattered irregularly and in loose 

 wisps, and partly of spined tylostyles projecting vertically from the substratum, 

 upon which their bases rest while their apices point outwards. 



Spicules: (1.) Tylota (Plate XL, fig. 5, a); smooth, slender, straight or nearly 

 so, with rather feebly developed oval heads and commonly with the shaft slightly 

 inflated (tylote) in many places at irregular intervals. Size about 0'148 millim. by 

 0-002 millim. 



(2.) Spined tylostyli (Plate XL, fig. 5, b) ; quite straight and abundantly spined all 

 over; with spherical head not much wider than the shaft, which tapers gradually 



