PUZZLES IN TAL^ONTOLOGY. 219 



of that order. They belonged to the four-gilled order of Cepha- 

 lopods, of which one representative — Nautilus — alone survives. 

 In a former paper I have alluded to the extraordinarily simple 

 structure of the eye in this ancient mollusc. The higher two- 

 gilled form — Bclcinniies — did not appear until the Triassic period. 



The Cambrian crustaceans — the Trilobites — belong to an 

 archaic type, which for ever disappeared in the Carboniferous 

 period. Yet, unlike Nautilus, they possessed highly developed 

 eyes. Two important features strike us in examining the Cam- 

 brian fauna. One is, that the forms of life, notwithstanding their 

 number and diversity, are all salt-water animals, there being a 

 complete absence of land and fresh-water forms. And the other 

 main fact is the absence of any vertebrated animal. It is hardly 

 necessary to say that the lower forms of vertebrates may have been 

 in existence even at this early period, but of this we can never 

 hope to have any proof. The lowest vertebrates are destitute of 

 any hard parts which could have been preserved as fossils. We 

 may, however, hope that in some of the still unexplored regions of 

 the globe there may yet be found strata intermediate between the 

 Laurentian and the Cambrian, where the first beginnings of six of 

 the great orders of organic beings may be found. 



An unaccountable break in the development of mammalian 

 life occurs during the Cretaceous period, but it does not equal in 

 mystery the profound break between the lifeless Laurentian rocks 

 and the Cambrian, full of representatives of six of the great 

 orders of the animal kingdom. 



With the Silurian fauna, we find the first indisputable represen- 

 tatives of the great group of vertebrates, but not until the upper 

 Silurian deposits are reached. We here meet with remains of two 

 of the lower orders of fishes — the sharks or dog-fishes {Elasnio- 

 branchii) and the bucklered Ganoids : the former still very 

 abundant in modern seas ; the latter, which include the sturgeon 

 and the alligator-gar, probably verging on extinction. 



It is a significant fact that prior to the introduction of these 

 low vertebrata, all the larger divisions of the Invertebrata had 

 come into existence. The earliest ///vertebrate air-breathers have 

 been found in Silurian deposits, a true scorpioid {Faleoplioucus) in 

 the U. Silurian de])osits of Sweden and Scotland, and an orthop- 



