OF THE BRITISH SPECIES. 75 



dria to such as have siHceous spicula as the basis of their 

 structure ; separating, however, from these the Tethya be- 

 cause of the peculiar arrangement of their spicula, and the 

 hemispherical form of their bodies. The number of spe- 

 cies described under each genus is as follows : 



Tethya, 



2 



Grantia, 



5 



Halichondria, 



- 18 



Doubtful species, 



- 14. 



Spoiigia, 



I 







I am enabled through the liberality of my friends and 

 fellow-students, to make some additions to this list ; but 

 notwithstanding its extent, the British shores cannot be 

 deemed favourable to the production of sponges, for the in- 

 digenous species in general show that their waters are un- 

 genial to the race by the rarity of the normal species, and 

 by the dwarfed si^e and compact structure of the others. 

 Of the genus Spongia we have at most but two incon- 

 siderable representatives, about a foui'th part of them are 

 calcareous ; but the great majority are secerners of silex, 

 which they, like skilful artificers, fashion into crystalline 

 needles and interweave with the parenchyma of their 

 bodies. 



Fig. 10. 



