1122 
DE. J. S. BOWEEBAJfK ON THE ANATOMY 
two or three compressed together. In some genera belonging to this suborder the arenar 
tion of the fibres is confined to the primary or radial ones, and the fibres of the secon- 
dary system are destitute of extraneous matters. In other genera they occur occa- 
sionally in the secondary system as well as in the primary one. In Stematumenia the 
primary fibres are frequently somewhat compressed, and are abundantly arenated. The 
smaller or secondary series of fibres are usually cylindrical, and most frequently without 
either grains of sand or spicula. Several of the common Bahama sponges of commerce 
belong to this suborder ; but the best type is the genus Stematumenia^ described by me 
in the Annals and Magazine of Natural History for December 1845, voL xvi. p. 406, 
plate 14. figs. 1 & 2. The genus may be characterized as follows: — 
Stematumenia, Bowerbank. 
Skeleton. Primary fibres solid, more or less compressed, containing a central axial line 
of spicula and grains of extraneous matters. Interstitial structures abundantly fibro- 
membranous. Part II. Plate XXVII. figs. 3 & 5, and Plate XXVIII. figs. 1 & 2. 
Suborder VII. Irregular and entirely areno-fibrous skeletons. 
Types, Bysidea fragilis, Johnston. 
Dysidea Kirkii, Bowerbank. 
The peculiarity of this suborder is, that the fibre of the skeleton is a full and complete 
but elongate aggregation of particles of sand, each separately coated by keratode, form- 
ing a series of stout anastomosing fibres, consisting of innumerable extraneous molecules 
encased by a thin coat of keratode. 
In Dysidea Kirkii, an Australian species, both the primary and secondary fibres of the 
skeleton are comparatively large, frequently exceeding half a line in diameter. In our 
British species, Dysidea fragilis, Johnston, the primary fibres are often as abundantly 
arenated as those of the Australian species, while the secondary ones are only partially 
filled with extraneous matter, and in this condition they are frequently more or less 
tubular. Part II. Plate XXVIII. figs. 3, 4 & 5. 
The structure and peculiarities of the above-named two species are described in detail 
in vol. i. p. 63, plate 6 of the Transactions of the Microscopical Society of London. 
On the Discrimination of the Species of the Spongiadce. 
One of the reasons why so little progress has been made in our knowledge of the 
Spongiadse is, that the generic and specific characters that are visible to the unassisted 
eye, such as form and colour, are in this class of animals remarkably uncertain and 
delusive, while all those that are definite and constant require not only a high degree 
of microscopical power to make them visible, but frequently also a peculiar mode of 
treatment to render them apparent even beneath the microscope. Thus it is with many 
