BRITISH SPONGIADiE. 337 



spicula of the skeleton are comparatively large and stout, 

 and they vary considerably in the amount of their spination ; 

 sometimes the spines are so few in number and so minute 

 as to be scarcely discernible, while in others they are 

 prominently and abundantly produced ; in almost all cases 

 they are absent at the middle of the shaft, but occasionally 

 the whole spiculum is entirely, but incipiently, spinous. It 

 is a singular feature in this sponge, that, in the interstitial 

 membranes, the retentive spicula are frequently absent for a 

 considerable space, and then a single localised group will 

 appear in which they are very abundant. 



43. Isodictya fimbriata, Bowerbank. 



Sponge. Massive, sessile. Surface smooth. Oscula 

 simple, dispersed, numerous. Pores inconspicuous. 

 Dermal membrane pellucid, abundantly spiculous ; 

 spicula acerate, without spines, dispersed or sub- 

 fasciculate ; abundantly and often profusely furnished 

 with dispersed bidentate bifimbriate equi-anchorate 

 retentive spicula, large and small. Skeleton. Strong 

 and coarse ; primary lines multispiculous ; secondary 

 lines irregular, mostly bi or trispiculous ; spicula 

 acuate, stout and rather short in their proportions ; 

 apically and basally spined ; occasionally entirely but 

 incipiently spined. Interstitial membranes. Tension 

 spicula acuate, long and slender, occasionally flexuous, 

 few in number; retentive spicula, bidentate biflm- 

 briate equi-anchorate, large and small, dispersed, very 

 abundant. 



Colour. Dried, nut-brown to dark purple. 

 Habitat. Shetland, Mr. Barlee and Rev. A. M. 

 Norman. 



Examined. In the dried state. 



I am indebted to the kindness and liberality of my late 

 friend, Mr. Barlee, for my first knowledge of this interesting 



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