1090 
DE. J. S. BOWEEBANK ON THE ANATOMY 
necessary to the determination of a genus, and occasionally we find one or more of these 
modes of organization entirely absent ; we may therefore consider them not as primary, 
but rather as secondary or auxiliary generic characters. 
I therefore propose to consider the varieties in the construction of the skeleton as the 
foundation or primary source of division into genera, and to dedicate that portion of 
the animal especially to that purpose, the auxiliary or secondary characters being 
resorted to only when required to aid and assist the primary ones ; and it is only to a 
very limited extent that they are in reality available. Thus the cloaca in the Order Cal- 
carea becomes a very important means of generic distinction, and in some cases in the 
Order Keratosa it is also a prominent character, while in Silicea it is generally absent. 
In some species of this order, as in Alcyoncellum, Polymastia, and Halyphysema, it 
assumes a normal character, while in several species of Halichondria, as in H. jyanicea, 
it assumes very striking proportions in excessively developed specimens, whilst in others 
it is either an occasional, uncertain, and progressive organ, or is altogether absent. 
The mode of propagation is also an uncertain character. Thus in Tetliea cranium we 
find it to be by internal gemmulation, in T. Lyncurium by external gemmules, and in 
other species of the genus no gemmules of any description have hitherto been detected. 
In Geodia, Pachymatisma, and Siyongilla the general structure and mode of disposition of 
the ovaria render them valuable auxiliary generic characters, but in other cases they 
are of little or no value. 
The intermarginal cavities are available as generic characters in Geodia and the 
nearly allied species; and in the same sponge the relative position of the connecting 
spicula form good distinctive characters in the genera Geodia, Ecionemia, and also 
some of the siliceo-fibrous sponges. In Alcyoncellum, Polymastia, and Geodia the 
position and appendages of the oscula are also available ; but generally speaking those 
organs are so mutable as to render them of little value as generic characters. 
The following tabular view of the arrangement I propose to adopt will perhaps render 
the details regarding the distinctive characters and natural affinities of the genera more 
readily comprehensible. 
