Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/466

Rh 442 MAMMALIA [CARNIVOKA. si;ie of the other teeth, in the very large extensile lips, and in other minor characters. The one species, M. labialus, the well- known Sloth Bear of India, feeds chiefly on black ants, termites, beetles, fruit, lionoy, &c. The great Cave Bear, the remains of which are found so abund antly in caves of Pleistocene age in Europe, was a true Ursus, and as much or more specialized as any existing species, as it had lost its three anterior premolars in the adult state, but in Ursus arvernensis and older species from the Pliocene they were all re tained. Still more generalized forms of Bears, presenting various degrees of transition towards a common Carnivorous type, are repre sented by the genera Arctotherium from South America and Hyeenarctos of Miocene strata of Europe and Asia, and others which are not far removed (at least in dental characters) from such primitive Dog-like types as Amphicyon. EXTINCT CARNIVORA OF DOUBTFUL POSITION. The discovery of fossil remains in Eocene and early Miocene formations both in Europe and North America, shows that numerous species of terrestrial carnivorous animals existed upon the earth during those periods, which cannot be referred to either of the sections into which the order has now become broken up. By some zoologists these have been supposed to be Marsupials, or at least to show transitional characters between the Didelphous and Monodelphous subclasses. By others they are looked upon as belonging altogether to the latter group, and as the common ancestors of existing Carnivores and Insectivores, or perhaps rather as descendants or relatives of such common ancestors, retaining more of the generalized characters than any of the exist ing species. They shade off almost insensibly into numerous other forms less distinctly carnivorous, to the whole of which, including the modern Insectivora, Cope (to whom we are indebted for our principal knowledge of the American extinct species) gives the name of BUNOTHERIA, those more specially related to the existing Carnivora forming the suborder Creodonta, which is divided into the five families, Arctocyonidse, Miacidaz, Oxy&nidse, Amblyctonidse, and Meronyehidae,. These are cases, however, in which the appli cation of the principles of classification adopted in the case of existing species, of which the entire structure is known, and which have become divided into isolated groups by the extinction of intermediate forms, is really impossible. If the generally accepted view of evolution is true, and the extreme modifications pass insensibly into each other by minute gradations (a view the palseontological proof of which becomes strengthened by every fresh discovery), there must be many of these extinct forms which cannot be assigned to definitely characterized groups. There are, however, some which stand out prominently from the others as formed on distinct types, having no exact representatives at present living on the earth. Of these the best-known is that named Hysenodon, of which, with the nearly allied Pterodon, many species FIG. 122. Dentition of Hyxnodon leptorhyncfius (Lower Miocene, France). The posterior molar is concealed behind the penultimate tooth. have been found both in Europe and America. They had the full number of forty-four teeth, grouped in the usual manner, and the incisors, canines, and premolars were formed upon the ordinary Carnivorous type as now seen in the Canidw, the fourth premolar above and the first true molar below being formed upon the &quot;sectorial&quot; plan, but the teeth behind these, instead of being tuberculated as in all existing Carnivora, repeated the characters of the sectorial, and also increased in size, especially in the lower jaw, from before backwards. They thus present some resemblance to the teeth of such carnivorous Marsupials as the Dasyuridse ; but, as the researches of Filhol have demonstrated, their milk dentition follows precisely the rule of existing placcntal heterodont mammals, and not that characteristic of the Marsupials. They show, moreover, none of the essential cranial modifications which distinguish true Marsupials. The curious American genus Oxsena seems to have been a specialized form of this type, and the European Proviverra or Cynohysenodon of Filhol) forms a complete transition between it and the Viverridaz. In Arctocyon primasvus, the oldest known Tertiary mammal, from the lowest Eocene beds of La Fere, department of Aisne, France, on the other hand, all the molars were tuberculated, and have been compared with those of the Procyonidse and also Gymnura, among the Insectivora. The small size of the brain of these early forms is not, as has been supposed, a special Marsupial character, but is common to the primitive forms of all groups of vertebrates. Mesonyx, from the Eocene of North America, seems to have been a very generalized form, with flat blunt claws, and long and slender tail. Cope makes the interesting suggestion that this may have been in the ancestral line of the Pinnipeds, but his state ment that the scaphoid and lunar bones of the carpus were distinct offers a decided difficulty to the acceptance of this view. SUBORDER PINNIPEDIA. These differ from the rest of the Carnivora mainly iu the structure of their limbs, which are modified for aquatic progression, the two proximal segments being very short and partially enveloped in the general integument of the body, while the third segment, especially in the hinder ex tremities, is elongated, expanded, and webbed. There are always five well-developed digits on each limb. In the hind limb the two marginal digits (first and fifth) are stouter and generally larger than the others. The teeth also differ from those of the more typical Carnivora. The incisors are always fewer than -f. The molar series consists generally of four premolars and one molar of very uniform characters, with never more than two roots, and with conical, more or less compressed, pointed crowns, which may have accessory cusps, placed before or behind the principal one, but are never broad and tuberculated. The milk teeth are very small and simple, and are shed or absorbed at a very early age, usually either before or within a few days after birth. The brain is relatively large, the cerebral hemispheres broad in proportion to their length, and with numerous and complex convolutions. There is a very short caecum. The kidneys are divided into numerous distinct lobules. There are no Cowper s glands. Mammae two or four, abdominal. No clavicles. Tail always very short. Eyes very large and exposed, with flat cornea. The nostrils close by the elasticity of their walls, and are opened at will by muscular action. The animals of this group are all aquatic in their mode of life, spending the greater part of their time in the water, swimming and diving with great facility, feeding mainly on fish, crustaceans, and other marine animals, and progress ing on land with difficulty. They always come on shore, however, for the purpose of bringing forth their young. They are generally marine, but they occasionally ascend large rivers, and some inhabit inland seas and lakes, as the Caspian and Baikal. Though not numerous in species, they are widely distributed over the world, but occur most abundantly on the coasts of lands situated in cold and temperate zones. The suborder is divisible into three well- marked families : the Otariidse or Sea Bears, which form a transition from the Fissiped Carnivora to the Seals ; the Trichechidae, containing the Walrus ; and the Phoddae or typical Seals. Family OTARIIDJE. When on land the hind feet are turned forwards under the body, and aid in supporting and moving the trunk as in ordinary quadrupeds. A small external ear. Testes suspended in a distinct external scrotum. Skull with post-orbital processes and alisphenoid canal. Palms and soles of feet naked. Otaria. Dentition: i f, c, p |, in 1 -~ ; total 34 or 36. First and second upper incisors small, with the summits of the crowns divided by a deep transverse groove into an anterior and a posterior cusp of nearly equal height ; ths third large and canine-like. Canines large, conical, pointed, recurved. Molars and premolars, usually, of which the second, third, and fourth are