114 ~~ Prof. W. Thomson on the “ Vitreous” Sponges. 
than the corresponding bone in any living Monotreme), whereof 
I beg to enclose drawings; the bone is seen from three dif- 
ferent points of view,—to which are added sketches of the same 
part of an Echidna hystrix slightly enlarged. ‘They may be 
figured, however, of the same size as the sketch, because I 
have before me the articulated skeleton of an Hchidna in which 
the humerus is fully as large. The fragment in the possession 
of the trustees of this Institution is a portion of the distal part 
of this bone; the articulating surface, which fits into the sig- 
moid cavity of the ulna, is perfect; and, from its peculiar 
structure, it cannot well be mistaken for that of any other 
known mammal. 
I have not yet seen any of the papers lately published by 
Professor Owen on Australian fossil remains; and as it is pos- 
sible that a fossil Hchidna is already described, I do not wish 
to name the present species; otherwise I should propose the 
specific term of H. Owenzi for it. 
I have the honour to be, 
Gentlemen, 
Your very obedient Servant, 
Australian Museum, Sydney. GERARD KREFFT. 
November 23, 1867. 
XX.—On the “Vitreous” Sponges. By Professor WYVILLE 
Tomson, LL.D., F.R.S.E., F.G.8., MR.LA. - 
[Plate IV.] 
THE classification of the PORIFERA is as yet extremely un- 
satisfactory. ‘This arises chiefly from the circumstance that 
the essential part of a Sponge, the sarcode sheet investing the | 
supporting framework, presents no visible distinctive charac” 
ters, being apparently nearly the same in physical properties 
and in chemical composition throughout the vi series. 
Characters must therefore be founded upon accessory and 
comparatively unimportant parts; and these exhibit, with few 
exceptions, so finely graduated a series of minute variations 
that it is difficult to employ them in the definition of orders 
and suborders. Except m a few cases, but little stress can be 
placed upon the external form of the sponge-mass, even as a 
specific character. Often the general appearance of a sponge 
is characteristic enough, and a practised eye can easily recog- 
nize it in almost all its stages of growth; but it is impossible 
to embody the impressions on which this recognition is based 
in a description, or even to convey them by the most accurate 
figures. Hence the extreme difficulty in naming a collection 
