200 INVERTEBRATE PALAEONTOLOGY 



variety developed by Feather-Stars, Starfish, Brittle- 

 Stars and Sea-Urchins is available in this country until 

 the present period is reached. In some measure this 

 scarcity of fossils in the Tertiary may be ascribed to 

 the reduction in massively-built types of Echinoderms 

 begun in Mesozoic times. Stalked Crinoids are rare, 

 and have usually retreated to abysmal depths. Their 

 eleutherozoic offspring are too delicate for normal 

 preservation. Modern Stelleroids are overwhelmingly 

 numerous, but most of them seem even less suited for 

 fossilization than their ancestors. The abundance of 

 Echinotds, which are eminently adapted for preservation 

 (whether massive rock-dwellers or flimsy burrowers) in 

 Cainozoic rocks of other districts no less than in modern 

 faunas, shifts the responsibility for the feebleness of the 

 British record on to physiographical conditions. 



Crinoid remains are always rare in the Tertiary. 

 Isocrinus (usually represented by columnals) occurs in 

 the London Clay, and the genus is the least rare of 

 living stalked types. Such Stelleroids as are known 

 are essentially modern in facies. 



Both Regular and Irregular Echinoids seem near to 

 their acmes in the modern fauna, and their tests occur 

 in rock-forming profusion in the Mediterranean region 

 and parts of North America. The Cidaroida show little 

 change, in structure or numbers, but they seem to have 

 betaken themselves to parts of the sea remote from 

 littoral belts (on which they flourished in Jurassic times). 

 The Saleniid series of Diademoida is comparably 

 retiring, and has dwindled to very small faunal pro- 

 portions. The Arbacioid series (with ambulacral 

 structure similar to that of Hemicidaris) is fully repre- 

 sented by Arbacia and Coelopleurus, the latter being 

 abundant in Indian Miocene deposits. Diadema (Cen- 

 trechinus) is apparently a reversionary member of its 



