98 J. G. WALLER ON AN UNDESCKIBED 



are accumulated layers, one over the other, which may start from a 

 base having another axis. 



Procuring a portion for microscopic examination, after giving it 

 a rigid scrutiny, I became convinced that it was the sponge to which 

 Dr. Bowerbank had given the name of Hymeniacidon macilenta, 

 the specific term being very expressive of its meagre condition or 

 habit. His description, however, in no way alludes to the 

 peculiarity to which I have drawn attention, or to its fascicular 

 arrangement of the spicules. The specimen, therefore, from which 

 he made his description, must have been so developed as to have 

 concealed this structure, which could scarcely, otherwise, have escaped 

 the attention of so good an observer. Let me, however, now refer 

 to the intimate characters of Hymeniacidon macilenta as set down 

 in Vol. ii., p. 176, of the '' British Spongiada?." 



It is described as thinly coating zoophytes, and in the living 

 condition to be of a bright scarlet colour. The skeleton spicules are 

 sub-clavate acuate, those of the membranes of the same character, 

 but longer and slender ; and amongst the latter a few tricurvate- 

 acerate. Retentive spicula, inequibidentate and dentato-palmate 

 anchorate, small, and few in number, and the latter also very 

 minute. One of the most characteristic forms, which are most 

 abundant, are contort bihamate spicules of a large size. 



It is unnecessary to give any further details. But we must now 

 turn our attention to another genus, which Dr. Bowerbank divided 

 from this of Hymeniacidon on account of some special characteristics, 

 which bear directly upon the subject in hand. Hymeniacidon is 

 specialised, as having its spicules disposed without any kind of 

 order on the membranes. It is obvious, therefore, that the sponge I 

 have been describing as H. macilenta cannot be consistently 

 considered as belonging to the genus thus defined. It is impossible 

 to say that the spicules are arranged without order. Yet tliere can 

 be no doubt, that it is the same as that above-named, but in a 

 different state of development. That this not only belongs to the 

 genus separated from Hymeniacidon under the name of Raphiodesma, 

 but must also be referred to a species described by Dr. Bowerbank 

 in Vol. iii., p. 230, I will now proceed to consider. 



The genus llaphiodesma is distinguished from that of Hy- 

 meniacidon in that its skeleton is composed of bundles of spicules 

 unconnected with each other, the fascicules being represented as 

 compacted at their centres, but free at each end. But this latter 

 character will require some modification, as I shall presently show. 



