158 PORIFERA. 



Heteradinellidce. So great was the profusion of Sponge-life during 

 Carboniferous times that thick beds of chert — as conclusively de- 

 monstrated by Hinde — were formed by the gradual accumulation of 

 siliceous spicules on the floor of the sea. These siliceous sponge- 

 beds are largely developed in the Carboniferous limestone and Yore- 

 dale beds of the North of England, Wales, and Ireland, and may 

 reach a thickness of more than fifty feet [in Ireland, a maximum 

 thickness of over 300 feet of these chert-beds has been observed]. 

 The highest Palaeozoic deposits — viz., the Permian — have hitherto 

 yielded few or no unequivocal remains of Sponges. 



Coming to the Mesozoic deposits, the Trias has not yielded a 

 large number of Sponges, and the great majority of the known forms 

 of this period belong to the Calcispongice, and are referable to the 

 extinct group of the Pharetrones. On the other hand, an enormous 

 number of Sponges are known to occur in strata of Jurassic age, 

 almost all the hitherto recognised types of this period belonging to 

 the three groups of the Lithistidce^ Hexactinellidce^ and Pharetrones 

 (Calcisfiongice). At the summit of the Jurassic system (in the Pur- 

 beck beds) we also meet with the first undoubted representative of 

 the fresh-water genus Spongilla. In the Cretaceous system, and 

 especially in its upper division, Sponges also occur in vast numbers, 

 the three groups above mentioned being those most largely repre- 

 sented. It is noticeable, however, that the aberrant Calcisponges 

 which form the group of the Pharetrones diminish in numbers to- 

 wards the close of the Cretaceous period ; while the groups of the 

 Monactinellidce and Tetractinellidce, have a fair representation. In 

 parts of the Cretaceous system, and particularly in the Lower and 

 Upper Greensand, occur beds of siliceous rock, which have been 

 shown by Hinde to have been formed by the accumulation of the 

 microscopic spicules of various types of Sponges. In some cases, 

 these spicules are loosely compacted together, and give rise to a 

 porous siliceous rock. More commonly, however, the spicules have 

 undergone partial solution in sea-water, and the dissolved silica thus 

 obtained has been subsequently redeposited, and has formed a 

 siliceous cement which has bound together the undissolved spicules 

 in a general cherty matrix. In such cases, the true nature of the 

 rock can only be recognised by an examination of thin sections 

 (fig. 51) under the microscope. 



There is, further, every reason to believe^ that the nodular flints 

 which form such a striking feature in the White Chalk have been in 

 reality produced by the solution of the skeletons of flinty Sponges 

 and other siliceous organisms, and the subsequent redeposition of 

 the silica thus obtained in a solid form. It is well known, namely, 

 that the siliceous Sponges of the White Chalk have usually had 

 their original siliceous skeleton more or less wholly dissolved out, 



