6 ELEMENTS OF PALAEONTOLOGY 
called fossil organism, Hozoon, occurring in gneiss, has been proved to be of 
inorganic nature. 
The Palaeozoic or Primary 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), mollusks and worms predominate, associated with a 
few Pelmatozoa, coelenterates, sponges, and poorly preserved algae. In the 
Silurian system all 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, mollusks, 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 totally 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 con- 
sists 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 Palaeozoic 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 mar- 
supial types. During the Triassic and Jurassic periods, vascular cryptogams, 
conifers, and cycads remain the dominant plant forms, dicotyledons not oceur- 
ring 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 hand, birds, and most 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. 
Palaeontology and Physical Geography.—Not only do fossils con- 
stitute the very foundation of historical geology, but they furnish us in 
addition with invaluable information respecting the origin of the rocks in 
which they occur, the former distribution of land and water, climatal 
conditions, and the laws of geographical distribution that have prevailed 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 
