GENERAL OBSERVATIONS. 397 



type peculiarities of another, where this intermediate position of 

 the species is not apparent. Thus, in the age of Mollusks, perhaps 

 the most abundant form of Crustacean was the Ostracoid, which 

 had a bivalve shell like the Mollusks. 



None of the examples of comprehensive or undivided types have been drawn 

 from Mollusks. They are, in fact, less marked in this sub-kingdom, because all 

 its several grand divisions were represented in the first appearance of animal 

 life, even to the highest, that of Cephalopods. It started as an unfolded type. 

 There were even many of the minor subdivisions in the early Silurian. The 

 siphonated Conchifers — the higher group — commenced with the Potsdam period 

 (p. 191), if the genus Conocardium is rightly referred to the Cardium family; 

 Pteropods probably passed their period of culmination in the Silurian; Brachio- 

 pods, theirs, in the later Palaeozoic ; and Cephalopods, theirs, in the Mesozoic. 

 Gasteropods were less well represented than the other tribes, and these have 

 their fullest display in the present age. The Cephalopods, however, may 

 properly rank among comprehensive types ; they combine in the Molluscan 

 structure the large perfect eyes and other senses of the Vertebrates, and in 

 some of them an internal bone, along with great activity and strength, — cha- 

 racteristics not typical of Mollusks. They were the princes of the Palaeo- 

 zoic world until the appearance of Vertebrates, — the type they partly fore- 

 shadowed. 



(4.) The comprehensive types of early time become nearly or quite 

 extinct with the progress of the system of life, while the types which 

 were foreshadowed by them or partly comprehended in them are 

 long afterwards perpetuated. 



Trilobites, Lepidodendrids, Sigillarids, Calamites, become extinct 

 in the Carboniferous age, Cystideans in the Lower Devonian. It 

 will be found that the Ganoid fishes and Labyrinthodonts are also 

 examples under this law. The small Lycopodia of our woods 

 are the only representatives of the great Lepidodendrids of the 

 Coal era, while ferns, the typical Acrogens, and Conifers, the typi- 

 cal G-ymnosperms, are yet abundant in species. The Trilobites 

 were accompanied by typical Entomostracans, the Eurypteri and 

 related species, before the close of the Silurian ; and, when disap- 

 pearing in the Carboniferous age, the Tetradecapods, foreshadowed 

 in the Trilobites, were already in the waters. 



2. Exterminations. — At the close of each period of the Palaeozoic 

 ages there was a general extermination of the living species, which 

 was nearly, and sometimes quite, complete. Again, as each epoch 

 terminated there was an extermination of life, but in most cases 

 much less general. With the transitions between strata of different 

 kinds in the course of an epoch there were usually some extermi- 

 nations ; and even in the passage from layer to layer the extinction 

 of one or more species took place. In a corresponding manner 



