SPONGIDA. 69 



globular sponge, provided with a cup-shaped cavity, on which 

 the oscula open, whilst the pores open upon the external sur- 

 face of the sphere. It was a calcareous sponge, enjoying a 

 free mode of existence, and it presents points of decided 

 affinity to the recent genus 

 Grantia. In the Upper Silurian 

 Rocks occur many sponges, -,vy 



one of the most interesting .^ - ^p 



genera being Amphispongia, '. ''.'' ^ 

 comprising calcareous sponges, 

 oblong or sub-clavate in shape, 



containing a central cavity, and / 



probably opening above by a 

 single osculum. On this hori- 

 zon occurs also the genus Favo- 

 spongia ; and here, as well as in 

 the Lower Silurians, we have -. 



. ' tig. 16. Section of ..-,, 



the Singular genUS StromatO- pra-morsa, a lower Silurian 



pora, which will be spoken of (after 

 immediately. In the Devonian Rocks, the genus Sparsispongia 

 may be noted ; but the Carboniferous series has hitherto 

 proved singularly destitute of sponges. In the Permian Rocks 

 it is worthy of notice that there occurs a genus described by 

 Geinitz under the name of Spongillopsis, and believed by him 

 to be most nearly allied to the recent fresh-water sponges 

 (Spongilla}. This is a remarkable fact as bearing upon Pro- 

 fessor Ramsay's view, that the Permian Rocks were deposited 

 in inland waters of great extent. 



Leaving the Palseozoic series, the sponges of the Triassic 

 and Jurassic formations call for no special remark, except that 

 the latter series abounds with examples of the Calcispongia. 

 It is in the Cretaceous system, however, in which we meet with 

 the greatest development of the sponges. The flints which 

 form such a characteristic feature in the White Chalk are, in 

 some measure at any rate, "connected with the periodic 

 growth of large crops of the sponges " (Owen) ; and in some 

 sections of flint are found minute " spherical bodies, covered 

 with radiating and multicuspid spines," which have been 

 termed Spiniferites or Xanthidia, and are probably the " gem- 

 mules " of sponges. (By some, however, these are regarded 

 as being the " sporangia " of the Desmidice, an order of the 

 Protophyta.) The two most notable genera of Cretaceous 

 Sponges are Siphonia and Ventriculites. The Siphonice (fig. 17) 

 belong to a group of sponges termed Petrospongiada, from 

 their possession of a stony reticulate skeleton without spicula. 



