136 PROTOZOA. 



peculiar state of preservation, the spicules which formed the 

 original skeleton are now only represented by vacant spaces 

 or cavities, while the actual canals and intervals between 

 these are filled with the sandy matrix of the rock. The 

 lower part of the sponge (fig. 33, c) consists of a series of 

 radiating canals, separated by a series of calcareous bars or 

 bundles of spicules, the former now filled with the surround- 

 ing matrix, while the latter, being dissolved out, are repre- 

 sented by hollow tubes. The upper part of the sponge, on 

 the other hand, is composed of a matted mass of small spicules, 

 now represented only by cavities (fig. 33, d). These spicules 

 are so closely fitted together that their form is very difficult 

 to make out ; but, according to the apparently correct observa- 

 tions of Salter, they are composed of three rays, two of which 

 lie in the same plane and form a continuous line, while the 

 third springs from the point of junction of the other two, and 

 is directed at right angles to them. In the opinion of Salter 

 and Bowerbank, Ampliispongia is closely allied to the living- 

 genus Gfrantia, and, under any circumstances, there can be little 

 doubt as to its being truly an ancient type of the Calcispongice. 

 While all the living Calcispongice possess a skeleton com- 

 posed of disconnected spicules, there is no a priori reason why 

 we should not meet with fossil forms of an essentially similar 

 nature, but having a continuous or vermicidate skeleton, com- 

 posed of calcareous spicules primitively free, but ultimately 

 anchylosed so as to form a single and connected framework. 

 Many fossil sponges have been supposed to belong to this 

 now unrepresented category of Calcispongice with a continuous 

 skeleton, but most of them (including most of the forms for- 

 merly known as Petrospongiadce) have been shown by recent 

 microscopic researches to be truly siliceous sponges. 1 There 

 still remain, however, some fossils which were beyond all 

 question calcareous to begin with, and which cannot, with 

 our present knowledge, be assigned to a definite place in the 

 zoological series, unless we regard them, provisionally at any 



1 Since the above was written, Zittel has published a memoir in which he 

 refers a large number of Triassic, Jurassic, and Cretaceous Sponges to a special 

 group of Calcispongice in which the skeleton is fibrous and continuous, instead 

 of being composed of separate spicules. The principal types referred to this 

 group are the genera Peronella and Corynella. The apparently fibrous skele- 

 ton of these forms is stated to be really composed of spicula. 



