CAINOZOIC FAUNAS 199 



Poriferan life. Silicispongiae are very feebly repre- 

 sented in the Cainozoic, but their rarity as fossils must 

 be ascribed to the infrequent occurrence of deposits 

 formed in the deep water where these types abound at 

 the present day. The case of the Calcispongiae is differ- 

 ent The Pharetrones, which were of chief importance 

 in the Jurassic and Lower Cretaceous, are unknown 

 from the Tertiary, and seem to have suffered the fate 

 normal to vigorous groups. The Sycones still linger, 

 but in diminished numbers. The horny sponges (which 

 are unknown in the fossil state) are the most conspicuous 

 members of the phylum in modern faunas. 



(D) COELENTERATA 



Tertiary Coelenterata are very feebly represented in 

 Britain. Save for a few stunted Madreporaria (such 

 as Turbinolid) and occasional composite types (e.g. 

 Litharaea), they are hardly to be found in the Lower 

 Tertiaries; while Flabellum (PI. xv. fig. i) and Balano- 

 phyllia are the only important genera that lived in the 

 cooling climate of the Pliocene. Hydractinia sometimes 

 encrusts shells found in the Crag. On the whole, the 

 Tertiary Coelenterate fauna proves, as far as it goes, 

 to be closely akin to that still extant. 



(E) ECHINODERMATA 



The preference shown by Echinoderms for clean 

 surroundings, and their distaste for brackish water, cause 

 the British record of Tertiary forms to be lamentably 

 meagre. Save for a few (probably drifted) fragments 

 of Crinoids in the London Clay, associated with 

 occasional Ophiurids and burrowing Spatangids, and 

 an insignificant series of practically modern forms in 

 the Crag, no evidence of the remarkable abundance and 



