56 



KNOWLEDGE 



[March 1, 1893. 



bed " of the quarrymen. The survival of Trlr/onia in the 

 Australian seas alone affords a curious parallel to the per- 

 sistence of pouched mammals and monotremes on that 

 island-continent, and of the lung-fish in its rivers. 



The pearly nautilus, of which there are some three or 

 four species from the warmer seas, is likewise entitled to 

 occupy a place among " Hving fossils," since this group of 

 cephalopods has existel continuously to the present day, 

 from the epoch of the lower silurian, with a progressive 

 diminution in the number of species. It was long con- 

 sidered that the palseozoic nautili were congeneric with 

 the existing ones, but although this is probably not the 

 ease, the whole are so closely allied as to show a most 

 remarkable persistence of type throughout untold ages. 

 Nautili have, indeed, witnessed the incoming and the 

 decline of the whole group of ammonites, so characteristic 

 of the secondary rocks ; but the reason of the persistence 

 of the one type and the total extinction of the other type 

 appears entirely beyond our ken. 



The most remarkable instance of the persistence of type 

 is, however, afforded by the genus Lini/iihi among the 

 brachiopods, or so-cailedlamp-shells. Lingulas have oblong, 

 flattened, and somewhat nail-like shells, composed partly 

 of horny and partly of calcareous matter, and are attached 

 to foreign substances by a long muscular pedicle passing 

 out between the beaks of the two valves, which are 

 generally of a greenish hue. These molluscs range fi-om the 

 Cambrian — at the very base of the paheozoics — to the 

 present day, without any trace of generic modification, and 

 indeed, with no perceptible change. Moreover, the group 

 seems to be now as well represented in species as ever it 

 was, the total number of living forms being given in the 

 second edition of Woodward's " Mollusca " as sixteen while 

 the total of fossil species at that date was but ninety-one. 

 The lingulas are, therefore, the very oldest animals at 

 present in existence. They are, however, run somewhat 

 close by two other genera of brachiopods respectively 

 known as Discina and Cnuwi, both of which range from 

 the lower silurian or ordovician epoch to the present day. 

 The parallelism in this respect is not, however, so close as 

 it might at first sight appear, since the lower pah^ozoic 

 representatives of both the latter genera are subgenerically 

 distinct from their living analogues. While the first of 

 these two genera has some ten living species, the latter 

 possesses but five, and both had a large number of 

 palaeozoic representatives. 



It is, perhaps, almost superfluous to add that the whole 

 of the brachiopods are a waning group, although the types 

 known as rhynchonellas and terebratulas have been ascer- 

 tained, of recent years, to be more numerously represented, 

 both as regards species and individuals, than was formerly 

 considered to be the case. 



The so-called stone-lilies, or crinoids — near relatives of 

 the famUiar star-fish, but attached, in the young condition 

 at least, to the sea-bottom by a jointed stem— hkewise 

 constitute a group in which by far the greater number of 

 types are totally extinct, although a few survive to merit 

 the title heading the present article. The stone-lilies are 

 divided into two primary groups, of which one is totally 

 extinct, while the other, which does not extend backwards 

 beyond the secondary period, comprises the few existing 

 representatives of the class. The genus which may be 

 selected as especially worthy of the designation " living 

 fossil," is Pentmiinm, so named from the pentagonal form 

 of the discs of the stalk — so familiar to all who have 

 studied the fossUs of the British secondary rocks. The 

 pentacrinids were originally named on the evidence of 

 certain species from the British lias, which attained a 

 height of several feet, and flourished in extraordinary 



profusion on the old sea-bottom, as the reader who cares 

 to piy a visit to the fo35il galleries of the Natural History 

 Museum may see for himself. For some time the genus 

 was only kaown from the secondary rocks, but eventually 

 a miuute species (P. Ewopcis) was fouud living ia deep 



Fig. 2. — A Living Pentacrinid. 



water otf the Irish coast ; while in 175.5 a much larger 

 form (P. cajiut-iiu'dmir) was discovered in the West Indian 

 seas. For a long period these living crinoids were supposed 

 to be of extreme rarity, only a few examples being from 

 time to time procured. With the development of deep-sea 

 dredging it was, however, found that pentacrinids were 

 really abundant in certain localities, and several new 

 species (one of which is represented in the accompanying 

 figure) have been named of late years. Thus in the 

 summer of 1870 the late Dr. Gwyn Jeft'reys dredged 

 up quantities in the Atlantic ; while in the neighbour- 

 hood of the Aru Islands the late Professor Moseley 

 tells us that during the voyage of the ChaUmijer 

 more than thirty specimens of Pentncrinus were taken 

 at a single haul of the dredge in 500 fathoms of water. 

 Although their pyritized remains, which are so common on 

 the slabs of lias, would have indicated that these crinoids 

 must have been creatures of extreme beauty, no adequate 

 idea of their gracefulness would ever have been obtained 

 had the group not been represented in the living state. 



Pentacrinids are, however, by no means the only " living 

 fossils " belonging to tliis class of animals. For instance, 

 the smaller and simpler Rluz(x-ri)iux of the Atlantic is a 

 survivor from the eocene ; while both this genus and the 

 allied Bathijerinm, from depths reaching to 2100 fathoms 

 in the Atlantic, are near relatives of the extinct Boun/ueti- 

 crimis of the chalk. Moreover, all these forms are related 

 to the so-called Pear-encrinites {Ajiiocrinus), so common in 

 the middle Jurassic rocks of Europe; while these, again, 



