BRITISH SPONGIAD.E. 309 



they are usually elevated to the extent of one or two lines 

 above the general surface. The dermal network is close 

 and strong, and the areas more frequently assume the tri- 

 angular form than any other. The skeleton is very irre- 

 gular, in consequence of the great size and inequality of 

 the interinarginal and interstitial cavities ; but the struc- 

 tural characters of an Isodictya are unmistakeably present. 

 The normal form of the spicula is acerate, occasionally 

 somewhat fusiform, and very rarely an acuate or a sub- 

 cylindrical form may be observed. No traces of tension 

 spicula could be detected; and excepting in the inter- 

 marginal cavities the interstitial membranes were rarely 

 present, but the whole of the skeleton structure was abun- 

 dantly covered by sarcode. 



In the collection of Montagu's Sponges, in the possession 

 of Professor Grant, a fine specimen of this species is 

 labelled Spongia coalita, and Montagu has described it in 

 the Wernerian Memoirs, vol. ii, p. 80, as " rather com- 

 pressed, and of a brittle, corky nature." This description 

 applies much better to If. simidans than to H. coalita, for 

 which species Montagu has apparently mistaken his speci- 

 men. 



24. Isodictya dichotoma, Bowerbank. 



Sponge. Arborescent, pedicelled, surface smooth. Oscula 

 simple, dispersed, few in number. Pores visible. 

 Dermal membrane thin, pellucid, furnished with a 

 unispiculous network of short acerate spicula. Skele- 

 ton. Primary lines bi or trispiculous ; secondary 

 lines unispiculous ; rete the width of the length of one 

 spiculum ; very regular and distinct. Spicula acerate, 

 short and stout. Interstitial membranes. Tension 

 spicula acerate, short and slender. 



Colour. Light ochreous yellow, alive and dried. 

 Habitat. Hastings, a mile off shore. 

 Examined. In the dried state. 



