BIGSBY PALAEOZOIC ROCKS OF NEW YOEK. 283 



8. Medina Sandstone. — This set of strata gives us only one new 

 genus {ArthropTiyciis), and but 18 species of any kind of life. There 

 is, however, here and in the Clinton a sudden influx of marine plants 

 in vast masses, as if, at the commencement of a new phase of organic 

 life in the Middle Silurian stage, it was intended to provide a rich 

 supply of new aliment. 



9. ClintonJRocks. — It is here that the characteristic Brachiopods 

 Pentamerus, Chonetes, and StrojoTiodonta, first occur, together with a 

 single species of the genus Posidonia, the Gasteropods Cyclonema and 

 Platyostoma, and one or two Cephalopods ; but, beyond noticing the 

 great importance of the Pentameri in palaeozoic chronology, no farther 

 remark needs to be made on the fossils of these rocks. As very few 

 new forms are developed, as far as yet kno^Ti, in the Niagara and 

 succeeding sections, this part of our subject may now be closed, by 

 observing that the statements just made have exhibited a deter- 

 minate plan in the order of precedence in ancient life, — determinate, 

 because we see, from concurrent evidence, that the same method and 

 order has been pursued in Ireland, Wales, and on the continent of 

 Europe, as in the State of New York. 



§ 5. Increment and Decrement. — Palaeozoic genera and species 

 undergo " increment" and " decrement" (numerical increase and de- 

 crease) in passing through the two systems, Silurian and Devonian ; 

 or it may be that their numbers remain stationary throughout the 

 whole range of epochs. This law becomes very interesting, by pointing 

 out a remarkable agreement, in this respect, which obtains in the two 

 distant, and not altogether similar regions, of New York and Wales. 

 Out of 24 orders and genera, thus comprising nearly the total faunae 

 of both countries, there are only five instances of disagreement, 

 chiefly in the order of Gasteropods, namely in the genera MurcMsonia. 

 Pleurotomaria, Bellero^hon, and Orthoceras; and I am not aware 

 that the omission of any of the poorer genera at aU disturbs this law 

 of conformity, which, we must now add, extends to Eussia in its Tri- 

 lobites and Orthides, in its Haly sites catenulatus, and probably in more 

 forms still, judging from many resemblances in other points. 



In both New York and Wales, increment upwards and increment 

 downwards take place in the same 8 important groups of fossils 

 (16 altogether), while in 3 others the number of species remains 

 about the same throughout the series of Silurian rocks. Or the 

 same genera in the two countries now under comparison may de- 

 crease upwards and downwards from the same culminating point. 

 These facts admit of an important application, and show, as De 

 Verneuil has inferred on a kindred subject, that the law which pre- 

 sided over the numbers of animals is universal, and not dependent 

 on local occurences of any kind. Most of these facts are well exhi- 

 bited in Table XYI. 



§ 6. Duration of Invertebrate Life. — The fauna of this or of any 

 other period was intended and adapted to endure for an appointed 

 time, as consisting of finite beings. As one of the methods of fulfil- 

 ling this intention, great mortahty has prevailed at the end of each 

 epoch, preceded by a gradual diminution in the quantity and variety 



VOL. XV. PAET I. X 



