114 Prof. W. Thomson on the " Vitreous " SjJonges. 



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 Jiystrix slightly enlarged. They may be 

 figured, however, of the same size as the sketch, because I 

 have before me the articulated skeleton of an Echidna 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 

 structm-e, 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 Austi-alian fossil remains ; and as it is pos- 

 sible that a fossil Echidna is already described, I do not wish 

 to name the present species ; otherwise I should propose the 

 specific term of E. Qwenii for it. 



I have the honour to be, 

 Gentlemen, 

 Your very obedient Servant, 



Australian Museum, Sydney. GeeAED KeEFFT. 



November 23, 1867. 



XX. — On the ^^ Vitreous^'' Simnges. By Professor Wyville 

 Thomson, LL.D., F.R.S.E., F.G.S., M.R.I.A. 



[Plate IV.] 



The classification of the Poeifeea 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 whole 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 in 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 



