THE SPONGES. 123 



fibres, more often somewhat thinner. Meshes of the reticulum, rounded 

 at the corners; in superficial region, 170-250 /x in diameter; in interior, 

 200-500 IX in diameter. 



Spicules of the spiculo-fibres are closely packed in many rows, arranged 

 lengthwise and cemented together by a very small amount of spongin, 

 which does not form a coating over surface of the fibre. Spicules also 

 project irregularly from surface of the fibres. There are almost no free 

 spicules in the meshes. 



Many radial fibres are prolonged a short distance beyond the surface, 

 for the length or less than the length of a spicule, thus forming very 

 small projecting tufts. In addition a few separate spicules project radially 

 or obliquely, at points between the ends of the radial fibres, i. e. from the 

 fibres of the dermal reticulum between the nodes. But over much of the 

 surface these minute projections are lacking, surface being quite smooth. 



The dermal membrane is supported by a reticulum (surface view, Fig. 

 5, Plate 21) which is merely the outermost layer of the main skeleton. 

 The fibres measure 60-120 jx in thickness; meshes, rounded-polygonal and 

 170-250 fi in diameter. As in the main skeleton, the fibres in general are 

 compact and sharply outlined. Here and there spicules project from the 

 fibres well into the meshes, or cross them, but in the meshes in general 

 there are almost no free spicules. Usually in the nodes the crossing of 

 spicule tracts is discernible. 



Comparative. I have examined the type specimens of Petrosia similis 

 Ridley and Dendy, and find that P. similis densissima stands closest to var. 

 compacta (Ridley and Dendy, 1887, p. 12). The skeletal fibres of this variety 

 are not nearly so compact as in P. similis densissima. This statement applies 

 both to the surface reticulum and to the main skeleton as well. Ridley and 

 Dendy (1887) correctly say that " the skeleton fibre is by no means so com- 

 pact and well developed " as in P. dura (Nardo), whereas in P. similis den- 

 sissima the fibres are fully as compact as in P. dura (Specimen No. 1818 in 

 Berlin Museum f. Naturkunde, from Rovigno), and appear more so because 

 there are fewer spicules scattered in the skeletal meshes than in the Medi- 

 terranean species. Owing to the comparatively indistinct outlines of the 

 fibres in P. similis var. compacta, preparations of the dermal membrane and 

 radial sections present a marked difference to corresponding preparations 

 of P. similis densissima. Especially the internal skeletal reticulum, as seen in 

 radial section, is confused and indistinct as compared with subsp. densissima. 



