HARD WICKE'S S CIENCE-G OSS IP. 



251 



each tank, with the exception of the glass front, is 

 well pitched and covered with fragments of granite, 

 arranged as fancy dictated. The slate bottom rests 

 on the floor in front, at about four inches from the 

 glass, thus leaving a sort of gutter running the whole 

 length, and which is filled to a depth of about one 

 and a half inch, — one part with shingle, and the 

 remainder with sand. It is now more than six 

 months since the first two tanks were completed, and 

 they are in better condition than when freshly stocked. 

 The water is kept at the proper density by means of 

 two little specific gravity bulbs, so weighted, that one 

 rises to the surface directly the water becomes too 

 salt through evaporation, and the other sinks to the 

 bottom if too much fresh water be added. The 

 larger of these two tanks contains more than sixty 

 anemones, principally the common Actinia meson- 

 b}yanthenmm, although there are several Actinoloba 

 dianthus, Cereiis geinmaceiis and Sagartia bellis. The 

 other inhabitants consist of about a dozen Corynadis, 

 as many prawns, a starlet starfish, and a few common 

 periwinkles. Of course, the greatest difficulty is in 

 maintaining the balance between animal and vegeta- 

 ble life, and which is so essential to the well-being of 

 the former. Now, although there is not sufficient weed 

 to give oft" the necessaiy amount of oxygen, yet 

 the loss is made up by the bellows arrangement, 

 which forces a series of small jets of air through 

 the water, and, rising in the form of minute bubbles, 

 so give off their oxygen ; thus burning out any 

 impurities. I tried to keep a few "Opelets" 

 {Anthea cereiis) and succeeded in retaining them in a 

 healthy condition for about three months, when they 

 one by one sickened and had to be taken out : these, 

 together with a few prawns who died a natural death, 

 are all the losses I have sustained in my marine family, 

 whilst on the other hand may be enumerated the 

 frequent birth of anemones, of which there are now a 

 considerable number. Several of the prawns have 

 spawned ; but the young fry were either cannibalized 

 by their parents or became food for the anemones. 

 In their early days they form most interesting micro- 

 scopic objects and are easily preserved in glycerine. 

 One peculiarity noticeable concerning them is, that 

 they were invariably born in the night. The larger 

 prawns occasionally cast their skins, and when 

 fortunate enough to be a spectator of the process, 

 nothing is more interesting. For some hours before 

 the event takes place the prawn swims about apparently 

 seeking some dark shelter ; suddenly the poor thing 

 seems seized with a series of rapid convulsive jerks, 

 the head is drawn out of its shell and the body follows 

 after a few more jerks. The whole performance is 

 but the work of an instant, and then the fenceless 

 prawn, with a few languid flaps of its tail, steers to a 

 place of shelter, there to await the hardening of its 

 new skin, which is soon accomplished, and once more 

 it takes its place among its brethren, to fight the 

 battle of life. J. W. Worster. 



OUR COMMON BRITISH FOSSILS AND 

 WIIERT: TO FIND THEM. 



No. VI. 



By J. E. Taylor, F.G.S., &c. 



HAVING given the general zoological structures 

 and natural history habits of the Eiuriniks 

 both recent and fossil (see June No.), let us now turn 

 to their occurrence in the various 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 limestones oi 

 the Silurian, Devonian, and especially of the 

 Carboniferous formations, are 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 Encnnites. 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 oft 

 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 Encrinites 

 which sometimes for square miles together covered the 

 bottoms of the seas. In the clay bands which are 

 often intercalated in the Silurian and other limestones, 

 we have frequent geological evidence of how large 

 numbers of young Encrinites were killed by the 

 muddied water, and eventually buried in the muddy 

 sediments which had first destroyed them. The same 

 is often abundantly true of the fine clayey shales of 

 the Yoredale beds of Lancashire and Yorkshire, 

 where entire specimens, stems, heads, and fingers, of 

 frail but lengthy-stalked Encrinites are to be dis- 

 entombed in the most perfect condition. The best 

 place we know of, where these Encrinital remains are 

 to be found in "the Yoredale series, may easily be 

 discovered by following the bed of the river from 

 Hebden Bridge, in Yorkshire, towards Higher Green 

 Wood. The Yoredale shales crop out in cleanly cut 

 sections, owing to the river frequently denuding them 

 along the lines of natural joints. The geological 

 student will there find, strewn about, huge cubical 

 blocks of thin dark shale, crowded with fossils, such 

 as Goniafites, Orthoceratites, Nautili, and Encrinital 

 remains. He can while away many a i^leasant hour 

 in these secluded but exceedingly picturesque places, 

 with the murmur of the stream playing somnolent 

 music in his ears, and the most picturesque hilly 

 scenery ready to greet his eyes, whenever he thinks 

 proper to turn them away from the absorbing 



