156 LIVING FOSSILS. 



with the countless swarms in wliich casts of their shells 

 occur in the so-called "roach-bed" of the quarrymen. 

 The survival of Trigonia in the Australian seas alone 

 affords a curious parallel to the persistence 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 " living fossils," since this group of 

 cephalopods has existed 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 Palaeozoic nautili were congeneric with 

 the existing ones, but although this is j^robably not the 

 case, 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 

 ajjpears entirely beyond our ken. 



The most remarkable instance of the persistence of type 

 is, however, afforded by the genus Linguia among the 

 brachiopods, or so-called lamp-shells. Lingulas have 

 oblong, flattened, and somewhat nail-like shells, conrposed 

 partly of horny and jiartly 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 

 from the Cambrian — at the very base of the Palaeozoics — 

 to the present day, without any trace of generic modifica- 

 tion, 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 liviiiE;' 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, how- 

 ever, run somewhat close by two other genera of brachio- 

 pods respectively known as Disciiia and Crania, both of 



