The Genus Porosphcera, Steinmann. By George J. Hinde. 17 



genera the dermal layer is very fragile and easily removable 

 from the connected skeletal mesh of the body of the sponge, and 

 Dr. Doderlein* states that in the specimens of the latter genns, 

 which had evidently been dead some time before they were hooked 

 up from the bottom of the sea, every trace of the dermal layer had 

 disappeared. 



It seems to me, therefore, highly probable that in the various 

 forms of Poros])hcvra from the Chalk a spicular dermal layer 

 covered the surface originally, and its subsecpuent complete dis- 

 appearance from the very large majority of these specimens may be 

 attributed to the loose intermingling and the absence of organic 

 connection of its constituent spicules, whereby the crust became 

 liable to disintegration and removal soon after the death of the 

 organism. It is only owing to exceptionally favourable conditions 

 of preservation that some small fragments of the dermal layer still 

 remain on a few of these sponges. 



IX. Canal System. 



All the forms of Porosphmra possess a series of simple, straight 

 canals which, in P. globidaris and P. nuciformis, radiate in all 

 directions from a central point or area to the surface of the sponge 

 (pi. I., fig. 6), whilst in P. pileolus, P. patclliformis, also in P. 

 Woodivardi, they radiate upwards and outwards from the centre 

 of the concave base. The canals are closely arranged, uniformly 

 small, and of the same dimensions throughout their length : as 

 the sponge increases in size fresh canals are intercalated. The 

 canals are bounded by the spicular mesh-fibres and free intercom- 

 munication can take place in the small spaces between the fibres. 



In P. nuciformis, in addition to the radial canals of the interior, 

 there are simple, shallow, surface grooves, with intermediate, 

 slightly elevated, rounded ridges, which are directed meridionally 

 towards the summit of the sponge, where, however, there is no 

 special aperture. Generally there is but one pole towards which 

 the grooves converge (pi. 1., figs. 11-17), but in some rare speci- 

 mens there are two or more elevations which serve as centres (pi. I. 

 fig. 18). These grooves are but surface features, and they are fre- 

 quently so faintly marked as to be scarcely noticeable, but it 

 seems probable that they played some part in the circulation of 

 the sponge. 



In P. Woodwardi there are distinct, strongly marked, branch- 

 ing canals, which extend from one or more slightly raised peaks 

 down the sides of the sponge (pi. I., fig. 19). As in P. nuciformis, 

 there is no special aperture at the slightly projecting points where 



* Zool. Jahrb, x. (1893) p. 17. 

 Feb. 17th, 1904 c 



