BRITISH SPONGIAD.E. 301 



18. Isodictya Gregorii, Boiverbank. 



Sponge. Sessile, coating, fistulous. Surface uneven, with 

 ridges, prominently hispid. Oscula simple, small and 

 dispersed ; or large and terminal. Pores inconspicuous. 

 Dermal membrane pellucid ; sparingly spiculous ; 

 spicula same form and size as those of the skeleton. 

 Skeleton. Primary lines rarely more than bispiculous ; 

 secondary lines unspiculous ; rete width of the length 

 of one spiculum ; spicula acerate, rather long and 

 slender. External defensive spicula, same as those of 

 the skeleton. Tension spicula, same as those of the 

 skeleton, few in number. 



Colour. Dried, cream yellow. 



Habitat. Moray Frith, the Rev. Walter Gregor. 



Examined- In the dried state. 



I am indebted to the Rev. Walter Gregor, of Aberdeen, 

 for my knowledge of this species. The sponge coats about 

 a fourth of the interior surface of half of a bivalve shell 

 about an inch and a quarter in diameter, straggling in 

 irregular ridges over the surface it covers, and each of the 

 ridges appears to be fistulous. Numerous small oscula are 

 situated on the crown of each of the ridges, and the largest 

 of them is terminated by a single large osculum of about 

 the same diameter as that of the fistulous cavity of the 

 ridge. 



The hispidation of the surface is remarkably prominent 

 in proportion to the size and structure of the sponge. It 

 is produced by the projection beyond the dermal membrane 

 of the distal terminations of the primary radial lines of the 

 skeleton ; and these defences frequently extend to the 

 length of one and a half or two spicula beyond the dermal 

 surface, terminating with one or two slightly diverging 

 spicula. 



