ENCRINITES. 115 



Encrinites six or eight feet in length, the ossicles of 

 which were not a fourth part the diameter of those 

 abundantly met v/ith in the Carboniferous limestones 

 of Derbyshire. 



Having briefly considered the general zoological 

 structures and natural history habits of the Encrinites, 

 both recent and fossil, let us now turn to their dis- 

 tribution in the various geological formations. They 

 are by far the most abundant in the Primary rocks, 

 although they range upwards into the Secondary 

 strata, and frequently occur there in very large 

 numbers. But their distribution in the Primary rocks 

 is more general and abundant, and the types, or 

 generic forms, are more numerous than we find them 

 in the Secondary strata. Indeed, many of the lime- 

 stones of the Silurian, Devonian, and especially of the 

 Carboniferous formations, are largely, if not chiefly, 

 built up of encrinital remains. As limestones are 

 always indicative of what sailors call " blue water " 

 that is, water free from any muddy sediment and 

 perfectly clear it follows that such conditions must 

 have favoured the growth of Encrinites. In this respect 

 they were nearly related to the habits of reef-building 

 corals, to whom muddy water is an abomination and 

 sure death. A sudden surcharge of sea-water with 

 mud brought down by rivers will almost immediately 

 kill off millions of living coral polypes. And from 

 what we learn of the Stony Record, the same thing 

 happened in geological times to the immense groves of 



