104 J. G. WALLER ON AN UNDESCKIBED BRITISH SPONGE. 



seems to show that there is a natural connection between them. If 

 so, it would be a strong argument in favour of the general arrange- 

 ment of Dr. Bowerbank, as founded on natural conditions, which 

 distinguish one genus from another, yet ally them in a close bond of 

 union. 



To recapitulate then : this Sponge, referred to the genus Raphio- 

 desma, is but little removed from U. sordida, and but for the 

 absence of the tricurvate and bihamate spicules, and the possession 

 of long hair-like acerate spicules in the membranes, as it were in 

 substitution, might easily be pronounced to be the same. It is a 

 problem in the future to show why such a slight divergence exists, 

 or to what development it is due. 



Raphiodesma minima (J. G. Waller). 



Sponge, Sessile, coating. Colour, in living state, unknown. 

 Dried, cream-white. Pores inconspicuous. Oscula, dispersed, 

 simple, minute. Membranes pellucid, spiculous. Spicules acerate, 

 hair-like, flexuous, intermingled with minute inequianchorate den- 

 tato palmate spicules in great abundance, and a few of larger size 

 dispersed and congregated in rosettes. Skeleton. Spicules sub- 

 clavate, subfusiform, acuate, disposed in regular fasciculi, united at 

 the base, but spreading out at the apex, generally keeping to the 

 length of one spicule ; the bases and apices coincident. In de- 

 velopment these fasces seem to proceed in one direction only, at 

 nearly right-angles to a basal axis. The same description of spicules 

 are also found in the membranes, but few and dispersed. 



DESCRIPTION OF PLATE V. 

 Fig. 1. Early development of Hyyneniacidon macilenta (Bowerbank), from a 



pebble found at Torquay, showing the regular arrangement of fasces 



of spicules (40 diam.). 

 „ 2. Development of iJap^todesma wmima, exhibiting a similar arrange- 

 ment of fasciculi (40 diam.). 

 „ 3. Early development of the basal membrane, fasciculi, &c., of the 



same (90 diam.). 

 ,, 4. Skeleton spicule (200 diam.). 

 ,, 5. Hair-like spicules of the membranes (200 diam.). 

 „ 6. Rosette of anchorate spicules on one corner of the sponge ; the 



only one preserved (333 diam.). 

 „ 7. Form of anchorate spicule, showing also side view (333 diam.) The 



minute examples on the membranes do not differ materially in 



form, but are about half the size. 



