6 ELEMENTS OF PALEONTOLOGY 



The Faleozoic or Primarij Group comprises the Cambrian, Ordovician, 

 Silurian, Devonian, Carboniferous and Permian Systems, each of which is 

 made up of a great number of series, stages and zones. In the Cambrian 

 crustaceans, trilobites, brachiopods and worms predominate, associated with a 

 few echinoderms, coelenterates, sponges and poorly preserved algae. In the 

 Silurian system most classes of the animal kingdom are represented with the 

 exception of amphibians, reptiles, birds and mammals, while the flora still 

 consists of algae. Marine invertebrates are very abundant, especially crus- 

 taceans, moUusks, echinoderms and coelenterates, while only a few frag- 

 mentary fish-remains indicate the presence of vertebrates. All the species 

 and nearly all the genera have since become extinct, and belong for the most 

 part to extinct families and Orders. During the Devonian, Carboniferous and 

 Permian Systems, the same classes of animals continue as a body, but are 

 represented by frequently different families and genera. Fishes develop a great 

 variety of forms in the Devonian, amphibians (Stegocephalia) make their 

 appearance in the Carboniferous, and reptiles in the Permian. The flora 

 consists chiefly of vascular cryptogams, together with a few conifers and 

 cycads. 



The Mesozoic Group comprises three Systems — the Triassic, Jurassic and 

 Cretaceous. Many of the widely distributed Paleozoic types (Tetracoralla, 

 graptolites, crinoids, cystids, blastoids, brachiopods, trilobites) have either wholly 

 or in greater part disappeared, while others (cephalopods, lamellibranchs, sea- 

 urchins) are replaced by very different genera and families. Vertebrates are 

 remarkable for the gigantic size attained by amphibians (Labyrinthodonta) and 

 many reptiles, as well as for the wonderful variety of the latter. Birds appear 

 for the first time in the Upper Jurassic (Archaeopteryx), and mammals 

 towards the close of the Triassic, being represented by diminutive, probably 

 marsupial types. During the Triassic and Jurassic periods, vascular crypto- 

 gams, conifers and cycads remain the dominant plant-forms, dicotyledons not 

 occurring until the middle Cretaceous. 



The Cenozoic Group comprises the Tertiary and post-Tertiary or Quaternary 

 Systems. Among the invertebrates, ammonites, belemnites, Rudistae and 

 most of the Crinoidea have now passed away. Amphibians and reptiles have 

 greatly declined, and, like the invertebrates, are represented by still living 

 Orders. On the other band, birds, and particularly mammals, attain a 

 wide distribution ; the latter class branches out in such manifold variety, and 

 experiences such rapid development during Cenozoic time, that it alone 

 furnishes us with the principal index-fossils of this era. From now on the 

 flora consists chiefly of dicotyledonous plants. 



Paleontology and Physical Geography. — Not only do fossils con- 

 stitute the very foundation of historical geology, but they furnish us in 

 addition with valuable inform'ation respecting the origin of the rocks in 

 which they occur, the former distribution of land and water, climatal 

 conditions, and the facts of geographical distribution in former periods. By 

 means of analogy with recent species we are able in most cases readily to 

 determine whether fossil forms pertain to land, fresh, brackish or salt water 

 species, whence it is apparent under what conditions the strata were deposited. 

 The distribution of marine and fresh-water formations helps us to certain 

 conclusions respecting the extent of former seas and land areas. Deep-sea, 

 shallow water, and littoral deposits are readily distinguishable by means of 



