The Genus Porosphcera, Steinmann. By George J. Hinde. 11 



leaving an empty mould of silica, or it lias been replaced by 

 reddish ferric oxide, thus presenting a marked contrast to the 

 usual state of preservation of Porosphcera. Under exceptional 

 conditions, as in the Chalk at Flamborough, the matrix of the 

 interior of many of the specimens of Porosphcera, also consists of 

 powdery ferric oxide, but the exterior of these forms shows the 

 calcific mesh which readily distinguishes them from the siliceous 

 sponges in the same beds. 



Porosphcera likewise occurs either partially or entirely imbedded 

 within the flint nodules of the Chalk, and in this condition the 

 skeletal mesh is, in part, siliceous, as well as the matrix. In 

 some of these flint-enclosed specimens the structural details are 

 more distinctly shown than in the usual forms from the Chalk, 

 but in others, and even in parts of the same specimen, they are 

 only faintly visible. 



V. Form and Manner of Growth. 



A large proportion of the specimens of P. globular is are 

 approximately spherical without any indication of a point of 

 attachment. Small individuals of about 1 mm. in diameter show 

 that the rounded form prevailed in the early stages of the growth 

 of the organism, and there is but little variation in the shape of 

 the specimens up to 6 mm. in diameter. In larger specimens the 

 growth is less symmetrical, they become oval, loaf-shaped, cushion- 

 shaped, rounded or subangular at the bases, which are flattened 

 or even slightly concave (pi. I., figs. 1-4, 9, 10). In some instances 

 growth does not take place uniformly over all the surface, but in 

 layers which only extend over parts of the surface and overlap 

 each other (pi. I., fig. 2). Usually the entire outer surface is 

 dotted over closely with the minute apertures of the radial canals 

 bounded by the skeletal mesh, but in a very few rare examples 

 these are covered by small patches of a spicular dermal layer 

 (pi. I., figs. 7, 8). 



The specimens of P. nuciformis vary in form from nearly round 

 to melon- and pear-shaped ; they appear in all cases to have been 

 free. Their surfaces are covered-with simple, slightly raised ridges 

 or swellings with correspondingly open, shallow, intermediate 

 furrows, which extend longitudinally from the rounded to the 

 obtusely pointed summit of the specimens (pi. I., figs. 11-17). 

 In a few rare examples there are two or more areas from which 

 the ridges extend (pi. I., fig. 18). 



Numerous specimens of P. globularis, both of the spherical 

 and of the oval or cushion-shaped forms, and also of P. nuciformis, 

 are penetrated by cylindrical tubes, some of which extend quite 

 through, so that the specimen becomes a natural bead, whilst 

 others reach only to the central portion of the fossil or beyond to 



