150 NATURAL SCIENCE. March. 



species of crinoid contained on a single slab. We may infer from 

 this, and from the facts of the distribution of the crinoids of the 

 present day, that the later crinoids were more apt to live together in 

 large assemblages of a single species than were the earlier. The 

 results which this habit has had on their evolution are well-known ; 

 one result, for instance, was the extreme length of stem attained by 

 certain species of Liassic Extracrinns. 



Modern Shallow-water Deposits. 



We have previously alluded to the efforts of the Liverpool Marine 

 Biological Committee in investigating the deposits now being formed 

 on the floor of the Irish Sea (vol. vii., p. 229). Further details are given 

 in the Report to which we have just referred. We here meet again 

 with Professor Herdman's criticism of Dr. Murray's classification ot 

 submarine deposits into terrigenous and pelagic, terms which were 

 fully explained in Natural Science (vol. vii., p. 22 and p. 395; 

 1895). " I n °ur dredgings in the Irish Sea," says the Report, " where 

 the deposits ought all, from their position, to be purely terrigenous, we 

 meet with several distinct varieties of sea-bottom which are not formed 

 mostly from the waste of the land, and do not contain anything like 

 68 per cent, of silica, but, on the contrary, are formed very largely of 

 the remains of bottom-haunting plants and animals, and may contain 

 as little as 17 per cent, of silica. Such are the nullipore bottoms, and 

 the shell-sand and shell-gravel met with in some places, and the sand 

 formed of comminuted spines and plates of echinoids which we have 

 found off the Calf Island. These deposits are really much more nearly 

 allied in their nature, and in respect of the kind of rock which they 

 would probably form if consolidated, to the calcareous oozes among 

 pelagic deposits, than they are to terrigenous deposits, and yet they 

 are formed on a continental area close to land in shallow water. 

 Moreover, although agreeing with the pelagic deposits in being largely 

 organic in origin, they differ in being derived not from surface 

 organisms, but from plants (the nullipores) and animals which lived 

 on the bottom." Professor Herdman considers, therefore, that the 

 terrigenous group of Murray is an unnatural assemblage, containing 

 many deposits which have very little to do with the waste land. This 

 leads him to propose the following classification : — 



1. Terrigenous (Murray's term, restricted). — Where the deposit is 



formed chiefly (say, at least two-thirds, 66 per cent.) of 

 mineral particles derived from the waste of the land. 



2. Neritic. — Where the deposit is largely of organic origin, its cal- 



careous matter being derived from the shells and other hard 

 parts of the animals and plants living on the bottom. 



3. Planktonic (Murray's pelagic). — Where the greater part of the 



deposit is formed of the remains of free-swimming animals 

 and plants which lived in the sea above the deposit. 



Another feature of this investigation is, as we have formerly 



