EEPORT ON THE MONAXONIDA. 219 



support this view. Colour in spirit greyish -yellow. Texture firm. Surface fairly even. 

 Oscula; one {1 sometimes more) at the top of an oscular tube in the centre of the flat 

 surface. Pores (?). 



Skeleton. — The convex (lower) surface of the sponge is covered with a dense thatch 

 {vide woodcut, Fig. 8, th), of very long tylostylote or subtylostylote spicules, which, 

 radiating outwards and upwards from the centre, form, round the margin of the sponge, 

 the spicular fringe already mentioned. On the flat (upper) surface there is no such 

 thatch, but there is a thick ectosome (woodcut. Fig. 8,c) packed with tylostylote spicules, 

 which are for the most part vertically arranged. On the outside of this ectosome is collected 

 a thick stratum of foreign matter (woodcut, Fig. 8, d), through which the apices of the 

 tylostylote spicules often project for a short distance. Below the ectosome come the softer 

 tissues of the sponge (choanosome, vide woodcut, Fig. 8, ch), which are crowded with 

 vast numbers of closely packed, much shorter, tylostylote spicules ; these have no very 

 definite order except that they exhibit a tendency to an arrangement in thick, radiating 

 groups with the globular heads in the centre. 



Spicules. — Megasclera ; (l) the exceedingly long, straight, slender, tylostylote or sub- 

 tylostylote spicules forming the thatch and fringe. It is not easy to give the maximum 

 length of these, owing to the manner in which they break on attempting to separate them 

 for purposes of measurement, but we have found examples about 4 '7 mm. long, and this 

 seems to be no uncommon size ; their diameter is commonly about 0'02 mm.; the heads 

 are generally subglobular and not very strongly developed, and at the apex the spicule 

 tapers very gradually to excessive fineness. (2) The spicules of the choanosome ; these are 

 short, straight, stout, fusiform tylostyli, sharply pointed at the apex and with very well 

 developed, globular heads ; size about 0"3 by 0'016 mm. (In the Australian specimen 

 they are shorter than this.) (3) The spicules of the ectosome ; these are also tylostylote 

 or subtylostylote and are intermediate in size between the two forms above described. 



This species difiers markedly enough in external form alone from the original type of 

 the genus, Trichostemma hemisphsericum, Sars,' to be distinguished at a glance, for Sars' 

 species is very much larger, strongly hemispherical, and has the convex surface uppermost, 

 and provided, in the adult, with many oscula ; while the fringe of spicules slopes down- 

 wards, i.e., away from the osculum -bearing surface. The difi'erences between Triclio- 

 stemma sarsii and Trichostemma irregularis are indicated at the end of the description 

 of the latter species. 



The geographical distribution of Trichostemma sarsii is a very interesting one ; it 

 seems to be confined to very deep water, and was met with off" the Azores and in 

 Australian seas, thus giving a very wide range. There are, it is true, certain slight 

 differences between the specimens from the two localities, which have been indicated in 

 the description, but there is nothing to justify us in separating them specifically. 



1 Remarkable Forms of Animal Life, pt. i. p. 62. 



