628 DR. J. S. BOWERBANK ON THE SPONGIAD^E. [May 7, 



of a pin. The dermal crust rarely exceeds about one fourth or one 

 third of a line in thickness. The retentive spicula of the dermal 

 membrane are very minute, and require a microscopical power of 

 about 700 linear to clearly demonstrate their forms. Their greatest 

 diameters from opposite points of their radii is not more than about 

 one sixth or one fifth of that of one of the skeleton-spicula. They 

 are exceedingly numerous, and in many parts of the membrane very 

 closely packed together. 



The similar description of spicula on the interstitial membranes 

 are very distinct in size and structure from those of the dermal 

 membrane ; their extreme diameters from opposite points of their 

 radii average about two thirds of that of a large skeleton-spiculum ; 

 the radii are few in number and gradually attenuate to a sharp point. 

 When examined with a microscopical power of about 700 linear the 

 radii generally exhibit traces of incipient spination. These spicula 

 are very numerous, and some parts of the membrane are crowded 

 with them. 



The large cylindro-sphero-stellate spicula are sparingly intermixed 

 with those of the interstitial membranes ; they are much longer and 

 stouter in their structure, some of them being nearly as large as an 

 average-sized ovarium. The apices of their radii terminate hemi- 

 spherically, and they are profusely covered with large conical spines. 

 These spicula are very characteristic of the species. I do not recol- 

 lect having seen the same form before in any species of Geodia. 



Geodia tumulosa, Bowerbank. (Plate XL VII.) 



Sponge massive, sessile, tumulous. Oscula congregated in shallow 

 depressions on the apices of the tumulous projections, simple, nume- 

 rous. Pores — congregated porous areas conspicuous, very numerous ; 

 pores inconspicuous, few in number in each area. Dermis crusta- 

 ceous, thick, densely crowded with ovaria ; dermal membrane pellucid, 

 thin, crowded with very minute cylindro-subsphero-stellate spicula ; 

 tension-spicula acerate, minute, few in number. Connecting spicula 

 attenuato-patento-ternate, large and stout, not very numerous. 

 Skeleton — fasciculi polyspiculous ; spicula fusiformi-acerate, large 

 and stout. Interstitial membranes — tension-spicula acerate, small, 

 and few in number ; retentive spicula attenuato-stellate, rather large, 

 very numerous and very minute, few in number. Ovaria spherical, 

 slightly depressed, very abundant in the crustaceous dermis, and 

 numerously dispersed on the interstitial membranes. 



Colour in the dried state light ochreous yellow. 

 Hab. Honduras (Mr. Dyson); Jamaica (Mr. Gosse). 

 Examined in the dried state. 



The figure represents the upper surface of the specimen I ob- 

 tained from Mr. Dyson, who found it at Honduras. Its greatest 

 thickness is 2f inches. Portions of its surface are thinly coated by 

 a parasitical halichondroid sponge, and especially the underside of 

 it. It was originally based on the valves of a dead Pectunculus, 

 which is still attached to its under surface, in which it is partly 



