REPORT ON THE MONAXONIDA. 115 



Histoderma a2'>]pendiculatnm, Carter/ appears to resemble this genus more strongly 

 than any other described sponge, having a similar form (though the tubes on the 

 surface are very long), a similarly arranged skeleton, and stout sigmata ; but as the 

 chela is of a normal form, and the sponge possesses no tylote spicules, the resemblance 

 may have no classiiicatory importance. 



Sideroderma navicelligerum, Eidley, sp. (PI. VIII. fig. 9 ; PI. IX. figs. 5, 8, 9). 



1885. Crella navicelligera, Ridley, Narr. Chall. Exp., vol. i. pt. 2, p. 571. 



1886. Sideroderma navicelligerum, Ridley and Dendy, Ann. and Mag. Nat. Hist., ser. 5, 



vol. xviii. p. 348. 



Sponge (PI. VIII. fig. 9) hemispherical, adhering by its broad surface; consisting of 

 a very hard, dense, external rind (whence the generic name), about 1 mm. in thickness, 

 and of an underlying soft mass of tissues containing numerous microsclera and loose 

 megasclera but no fibre. From the surface of the sponge arise a great number of 

 papillse, about 6 mm. in height, some with a (contracted) osculum at the extremity. 

 Size of the specimen 44 mm. in diameter at the base by 31 mm. thick. Colour in 

 spirit pale, dirty yellow. Texture, externally very hard and dense ; internally soft, 

 pulpy, amorphous (possibly a good deal decomposed). Surface smooth, except for 

 the numerous papillse. Oscida on the summits of papillse. 



Skeleton. — No horny fibre is present. The main part of the skeleton lies in the 

 external rind. This consists of a very dense, felted mass of tylote spicules arranged 

 with some degree of order in two main directions parallel to the surface. The spicules 

 lie in layers, touchiug one another, the spicules of the same layer lying more or less in 

 the same direction and at right angles to the spicules of the layer above or below. In 

 a vertical section through the rind (PL IX. fig. 9) we sometimes see very plainly the 

 alternating layers of longitudinally placed spicules, and of transverse sections of the 

 spicules. Occasionally spicules are found more or less vertical to the surface. The 

 spicules of the external rind are aU dumb-bell-shaped (tylota), and this adds very 

 much to their efiiciency, for by the interlocking of the heads of such .spicules one 

 with another a far firmer structure is produced than if the spicules were simply oxea or 

 strongyla. 



Spicides. — (a) Megasclera; tylota, each spicule consisting of a long, cylindrical 

 shaft, with an oval head at each end (PI. IX. fig. 5, a, h). Length 0-28 to 0-595 mm.; 

 diameter of shaft in middle 0-0063 to 0-0126 mm. The shaft is thickest in the middle, 

 and tapers slightly and gradually towards the knob at each end. These spicules also 

 occur abundantly scattered about in the soft tissues beneath the rind, and it is from 

 these that the measurements given above are taken. (6) Microsclera ; these are exceed- 

 ingly abundant in the deeper tissues (the isochelte also occur in the riud) ; and vary 



I Ann. and Mag. Nat. Hist, ser. 4, vol. xiv. p. 220; pi. xiv. figs. 23-25 ; pi. xv. fig. 39. 



