FOSSIL AIAMMALIA. Most of the mammalian types of modern times can be traced back to early Tertiary time, but before that period only scat tered synthetic or generalized types are known. In the late Paleozoic (Permian) and the earliest Mesozoic (Lower Triassic) rocks are found re mains of the so-called Theriodontia and Gompho dontia REPTILE), which possess characters remarkably mammalian in expression. These late highly specialized reptiles may be the root stocks from which the most primitive ancestral forms of mammals of the Triassic and Jurassic were evolved. The mammals of the early Meso zoic were of very small size, not larger than rats, and, as they are known chiefly by their jaws and teeth alone, which are of very primitive type, they can with difficulty he assigned to any of the later groups of the class. Some of these fragmental remains cannot be definitely distin guished as either reptiles or mammals, for their teeth show characters which are possessed by both the anomodont reptiles and the undoubted primitive mammals. Such forms are Micro lestes of the European Upper Trias, Tritylodon of the Karoo formation of South Africa, and Ste reognathus of the English Lower Jurassic. The principal genera of undoubted prototherian mam mals (primitive Monotremata) are Plagiaulax, Bolodon.Ctenacodon, Ptilodus. and Polymastodon. These are known only by small jaws containing sharp incisor teeth, three or four closely set cut ting teeth (premolars), of which the fourth is the largest, and the outer edges of which are marked by oblique furrows (see illustration under DUCK BILL), and finally two to four small molars. These molars are multituberculate, and hence the above mentioned genera are grouped in an order called Multituberculata. The other order of prototherian mammals, the modern Monotre rnata, is separated from the first order by almost the entire space of Tertiary time. One of these creatures. the duckbill, though toothless in its adult state, betrays its origin from a nmltituber ciliate ancestor by having in its younger stages of growth broad flat teeth remarkably like those of the Gomphodontia and Multituberculata.
The second subclass of primitive mammals, the Metatheria. comprises a single order. the Mar supialia, and this is divided into two suborders. The more primitive Polyprotodontia are char acterized by the presence of four or five incisor teeth in each jaw, and include the majority of Mesozoic mammals of Europe and North Amer ica: also the opossums (Didelphidfe), which ap peared first in the Laramie (Upper Cretaceous) ; the marsupials ( Dasyurida- ) of Australia, which can be traced hack only into the Pleistocene: the bandicoots (Peramelidal of Australia: and the banded anteaters 1 Myrmeco bida9, of which no fossil forms have been found. All thee are animals of comparatively small size, and they present various adaptations of the marsupial type to particular conditions of exist ence. The most important Mesozoic genera of Polyprotodontia are Pbascalotherium. Amphithe
rium, Amblotherium, Triconodon, Dryolestes, and Spalacotherium, and several others chiefly from the Upper Jurassic rocks of England and Wyom ing. The remaining suborder of Metatheria, the Hiprotodontia, is characterized by the presence of two large incisor teeth in each jaw, and its fossil and recent forms are almost wholly con fined to Australia. This suborder contains the gigantic Diprotodon. its smaller ally Notothe rium, and the peculiar 'pouched lion' of Owen (Thylacoleo), all extinct and found in Pleisto cene deposits: also the modern wombats, kanga roos. and phalangers. fossil forms of which have been found in the superficial deposits of Aus tralia and South America.
Among the Eutheria the Edentata, Cetacea. and Sirenia are of uncertain origin. They began with nearly normal eutherian types in the Eo cene, and they present through the Tertiary de generating series of peculiar animals, The Eden tata began in the Eocene, and during the Tertiary they developed some odd gigantic beasts—Mega therium, Mylodon, 3Iegalonyx. and Glvptodon (q.v.), the latter with a massive coat of armor plate. From these giant ancestors descended the comparatively small modern sloths, anteaters. and armadillos. The Cetacea is a group of mammals which through degeneration from the normal mammalian type has become peculiarly special ized and adapted to a fish-like habit of life. The earliest form in the Eocene, Zeuglodon, though distinctively fish-like, has, among other char acters typical of terrestrial mammals, the car nivorous type of dentition.
The hoofed mammals appear in the lowest Tertiary deposits of North America as forms (suborder Condylarthra) that can with difficulty he distinguished from the earliest carnivores. These early ungulates (typified by Phenacodus from the Lower Eocene of Wyoming) have five useful toes on their plantigrade feet and each toe has a small hccf. The brain is small and slightly convoluted, and the teeth are of a generalized type with low crowns and many tubercles. From the primitive Condylarthra seem to have been evolved all the later and more specialized forms of ungulates. The Perissodactyla comprise the extinct Titanotheroidea, large horned animals of Eocene and Miocene time (see 31) ; Hippoidea, the horses and their Eocene allies the paleotheres (gee HORSE, FOSSIL) ; Tapiroidea, in cluding the tapirs and their extinct Eocene and allies the Lophiodontid;e: Rhinocero toidea. the rhinoceroses, with a host of extinct ancestors reaching back into Eocene time: and the Chalicotheroidea, a peculiar group related to both the Peri sodac ty la and the Edentata. The Amblypoda include five-toed ungulates of large size that lived during Eocene time, of which well known genera are Coryphodon and Dinocerus. Artiodactyla, or even-toed ungulates, include the modern pigs. hippopotamus. camels. deer, cattle, etc., with long series of forms ancestral to these ranging back through Tertiary time. (See