The Genus Porosphccra, Steinmann. By George J. Hinde. 15 



incorporated with the mesh-fibres, so that they would be hardly, if 

 at all, recognisable in microscopic sections of the interior of the 

 organism. In older specimens of Porosphcera the mesh-spicules are 

 frequently considerably larger and thicker than in the young 

 individual just referred to (pi. II., fig. 9). 



In sections of specimens of P. globularis and of P. pileolus from 

 South Croydon, the free apical spicular rays near the surface are 

 seen to be furnished with stout lateral prickles, closely resembling 

 those in Plcctroninia Haiti (pi. II., figs. 5, 6). 



VII. Basal Layer. 



The hollow inverted cup- or thimble-shaped bases of P. pileolus, 

 and the more open saucer-like bases of P. patclliformis, show con- 

 centric lines or bands, which mark intervals of growth (pi. I., 

 figs. 20a-24a) ; and these are lined by very slender thread-like 

 and occasionally wavy spicules, disposed generally parallel with 

 each other, in the direction of the margin of the cup. These 

 spicules are so fine and closely set that it is difficult to determine 

 whether they are simple rods or rays of three-rayed spicules ; but 

 in some few specimens there is a thin exterior layer of straight, 

 slender spicular rods, extending downwards and outwards like 

 thatch on a» roof (pi. I., fig. 26a). 



In P. Woodwardi the base is small, concave and rugose, and its 

 spicular characters are obscured. 



VIII. Spicular Dermal Layer. 



With a few rare exceptions, the outer surface of Porosphcera, 

 even of well-preserved specimens which have had the chalky 

 matrix carefully removed, only shows the skeletal fibres and the 

 projecting apical rays of the spicules described above. But in the 

 exceptional examples, the usual skeletal mesh is covered in places 

 with a layer or crust of so different a character and appearance that 

 at first sight it might have been taken for an encrusting sponge 

 which had settled and grown on the outside of the Porosphozra. 

 This dermal layer appears as a whitish crust, in some instances 

 thin and smooth, in others of measurable thickness, uneven, and 

 with occasional small projections (pi. I., figs. 7, 8). Examined 

 directly under the Microscope, it is seen to consist, in the majority 

 of cases, of an agglomeration of minute three- and four-rayed 

 spicules, with an admixture of simple rods so intimately and 

 irregularly mingled together that it is difficult to distinguish the 

 individual forms. These spicules seem to have been originally quite 

 free and not connected together organically in any way (pi. II., 

 fig. 7). In one or two specimens the outer surface of the dermal 



