OF THE SPONGIAD^E. 171 



Both the type specimens of Geodia in the Museum at 

 the Jardin des Plantes appear to have had large central 

 cavities, but I have not found similar excavations in other 

 species of the genus excepting in one instance, a Geodia 

 from Port Elliot, Australia ; the internal surface in each of 

 the three cases presents precisely the same appearance, a 

 simple irregularly matted surface of spicula and membranes 

 without any thickening of the tissues, and differing in no 

 respect from the surfaces of any of the smaller internal 

 cavities of the sponge. I am therefore inclined to consider 

 such excavations as abnormal occurrences, which are not en- 

 titled to be considered as of either generic or specific value. 

 Fig. 354, Plate XXVIII, represents a section at right 

 angles to the surface of Geodia Baretti, Bowerbank, MS., 



a, a, longitudinal sections of two of the intermarginal 

 cavities ; b, b, the basal diaphragms of the intermarginal 

 cavities ; c, c, the imbedded ovaria forming the dermal crust 

 of the sponge ; d, d, the large patento-ternate spicula the 

 heads of which form the areas for the valvular bases of the 

 intermarginal cavities ; e, c, recurvo-ternate spicula within 

 the summits of the great intercellular spaces of the sponge ; 



f,f, portions of the interstitial membranes of the sponge 

 crowded with minute stellate spicula ; y, y, portions of the 

 secondary system of external defensive spicula X 50 linear. 

 See also Figs. 301, 302, Plate XIX. Fig. 301 represents 

 a small portion of the inner surface of the dermal crust 

 Geodia Barretti with two of the valvular membranes of the 

 proximal ends of the intermarginal cavities ; a, valve closed ; 



b, valve partly open; c, portions of the patento-ternate spicula 

 imbedded in the tissues and forming the areas for the val- 

 vular terminations of the intermarginal cavities X 50 linear. 



PACHYMATISMA, Bowerbank. 



Skeleton composed near the external surface occasionally 

 of short fasciculi of siliceous spicula, disposed in lines 

 at about right angles to the surface of the sponge. 

 Central portion of the sponge unsymmetrical. Der- 



