286 Royal Society : — 



and are held together by a layer of elastic sarcode, which invests 

 all the spicules and all their branches. Between the rays of the 

 spicules, over the whole surface, the sarcode forms an lUtimate and 

 very delicate network, its meshes defining minute inhalant pores. 



At the top of the sponge there is a large osculum, about 3" in 

 diameter, which terminates a cylindrical cavity, which passes down 

 vertically into the substance of the sponge to a depth of 5" 5'". 

 The walls of this oscular cavity are formed upon the same plan as 

 the external wall of the sponge ; and the stars, which are even more 

 conspicuous than those of the outer wall, are due to the same 

 arrangement of spicules of the same form. The ultimate sarcode 

 network is absent between the rays of the stars of the oscular surface. 

 The sponge-substance, which is about 2" in thickness between 

 the oscular and the outer walls, is formed of a loose vacuolated 

 arrangement of bands and rods of greyish consistent sarcode, con- 

 taining minute disseminated granules and groups of granules of 

 horny matter, and minute endoplasts. 



Towards the outer wall of the sponge the sarcode trabeculse are 

 arranged more symmetrically, and at length they resolve themselves 

 into distinct columns, which abut against and support the centres 

 of the stars, leaving wide, open, anastomosing channels between 

 them. The sarcode of the outer wall, and that of the wall of the 

 oscular cavity, is loaded with minute spicules of two principal forms — 

 quinqueradiate spicules with one ray prolonged and feathered, and 

 minute amphidisci. 



Over the lower third of the body of the sponge, fascicles of enor- 

 mously long delicate siliceous spicules pass out from the sarcode 

 columns of the sponge-body in which they originate, through the 

 outer wall, to be diffused to a distance of not less than half a metre 

 in the mud in which the sponge lives buried ; and round the osculum 

 and over the upper third of the sponge, sheaves of shorter and more 

 rigid spicules project, forming a kind of fringe. 



The author referred all the sponges wliich were found inhabiting 

 the chalk-mud to the Order Porifera Vitrea, which he had defined 

 in the ' Annals and Magazine of Natural History ' for February 

 1868. This order is mainly characterized by the great variety and 

 complexity of form of the spicules, which may apparently, with 

 scarcely an exception, be referred to the sexradiate stellate type, a 

 form of spicule which does not appear to occur in any other order 

 of sponges. The genus Holtenia is nearly allied to Hyalonema, 

 and seems to resemble it in its mode of occurrence. Both genera 

 live imbedded in the soft upper layer of the chalk-mud, in which 

 they are supported, — Holtenia by a delicate maze of siliceous fibres, 

 which spread round it in all directions, increasing its surface without 

 materially increasing its weight — Hyalonema by a more consistent 

 coil of spicules, which penetrates the mud vertically and anchors it- 

 self in a firmer layer. 



It appeared to the author and to Dr. Carpenter, who had had 

 their attention specially directed to this point as bearing upon the 

 continuity and identity of some portions of the present calcareous 



