BIGSBY PALEOZOIC BOCKS OF NEW YOBK. 261 



the Silurian system the especial Annelid- or Worm-joeriod ; for the 

 size and numbers of these creatui'es appear to have then been 

 larger by far than at any subsequent time. Judging from the sedi- 

 mentary habitats of the Crustacea, Brachiopoda, Dimyaria, Gastero- 

 poda, Heteropoda, and C^p^aZopoc^a of Table YI., the zones of marine 

 life established for the present epoch by the lamented Professor Ed- 

 ward Forbes cannot be applied with rigour to the Silurian seas. We 

 see that the Monomyaria and Gasteropoda are more deep-sea than lit- 

 toral ; and that Heteropoda abound in shallows and moderate depths, 

 becoming almost extinct elsewhere. We see that the remains of 

 fish are eminently littoral. Other interesting facts may be gathered 

 by consulting the Tables. 



We shall now enter into a few details on the organic relations of 

 the great sedimentary elements — calcareous, argillaceous, and arena- 

 ceous, successively. 



a. Calcareous strata. — A stratum highly charged with lime (but 

 having Kttle magnesia) usually abounds with Testacea ; and they 

 vary in number and perfection according to the greater or less ad- 

 mixture with other matters. When these strata contain alumina, 

 we have the maximum of early palaeozoic life. But if they become 

 wholly carbonate of lime, comparative barrenness follows in Wales ; 

 for fossils only occur 37 times in pure limestone, that is, about ^b-^h 

 as often as in argillaceous limestone, and forming ^th part of Silurian 

 life in Wales. In the Kew York Table of Habitats, a similar dis- 

 position prevails, the ratio in the former case being as one to three. 

 With a more accurate Table, the deficiency of evidence of Silurian 

 life in pure limestone will probably be yet more striking. 



The Pentamerus Knightii of Aymestry is a fine example of the 

 quick feeling of a mollusc (J. Phillips). If the rock lose its lime, 

 and become altogether argillaceous, the animal disappears ; but if, 

 as at Sedgeley, the lime returns, with it also returns the mollusc. 

 Or growth may be affected. Thus, among the Brachiopoda, Tere- 

 bratula princeps, found near Mnienian (Bohemia), is, like Phacops 

 fcecundus, double the usual size. So it is with the gasteropod Natica 

 in that locality, and also with others*. 



Barrande f finds that scarcely any two beds of limestone are 

 equally capable of preserving organic remains ; each layer and each 

 spot sheds its own influence over the fossil. The envelopes of some 

 Testacea are totally dissolved in certain calcareous masses ; but not 

 so in others. A red oxide of iron or a thin film of clay is a great 

 preservative of the Trilobite: mere fineness of texture, as in the 

 Bohemian schists, is not sufficient. Animal existence becomes sud- 

 denly multitudinous in the aluminous limestones of the New York 

 group B (Trenton) ; and at about the same epoch in Wales a nearly 

 equal abundance and variety of life burst upon our view, increasing 

 in both hemispheres, on the whole, as the long succession of fos- 

 siliferous strata overlie each other. The Annelida of New York 

 are calcaricolous, with three exceptions. In Wales also they are 

 mostly in calcareous strata, but only two in pure limestone. Of 

 * Barrande, Syst. Sil. p. 297. t Op. cit. p. 294. 



