Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/399

Rh MOXOTREMATA.] MAMMALIA 377 their general characters to those of the English Purbecks, some being even considered to be generically identical. Both polyprotodont and diprotodont types are represented, the latter by a species called by its discoverer Ctenacodon serratus, very closely allied to Plagiaulax. It will be of very great interest to know the mode of succession of the teeth of those early mammals, as it may throw some light upon the question of the relation of the succession of teeth in mammals generally with the same process in the inferior classes of the Vertelrata. There is, however, as yet very little, if any, reliable evidence upon the subject, but such as there is rather points to the fact of an absence or very feeble development of the diphyodont condition, resembling that of modern Marsupials. If this is so, it may lead to the somewhat startling conclusion that in the transition from the lower vertebrate to the mammal, by whatever process it took place, the indefinite reproduction of the teeth of the former was lost, and that a monophyodont condition supervened, which was again superseded by the peculiar definite diphyodont mode of succession characteristic of the most highly organized mammals. There is nothing yet known in the structure of these small mammals of the Mesozoic ages of the world to connect them with the surviving representatives (the Monotremes) of the hypothetical Prototkeria ; but whether their position was among the Metatheria or Eutheria, or whether they represented generalized forms from which both these branches have been derived, it is impossible at present to say. To avoid the difficulty of endeavouring to find places for them in any of the existing groups, Marsh proposes 1 to found two new orders for their reception Pantotheria for those of the polyprotodont or insectivorous type of dentition, and Allotlieria for Plagiaulax and its allies. The former may be convenient, but it is scarcely advisable to separate the latter ordinally, as long as we continue to place Phascolomys and Thylacinus, Cliiromys and Lemur, Trichecus and Phoca in the same orders, for Plagiaulax and Amphitherhim do not differ in the characters of their jaws and teeth more than any of these examples, which show how much the dentition may be modified with com paratively little general diversity of structure. This scanty evidence of mammalian life must bear a very small proportion to that which doubtless existed during the greater part of the vast Mesozoic period. The Cretaceous formations have as yet yielded no trace of the presence of animals of this class; but the number and variety of species met with in the earliest Tertiary forma tions, when already differentiation into most of the existing leading divisions had taken place, strikingly proves the imperfect state of our geological record during the immedi ately antecedent ages of the world. CHARACTERS OF THE DIFFERENT ORDERS AND FAMILIES AND OF THE PRINCIPAL FORMS OF THE MAMMALIA. SUBCLASS PEOTOTHEBIA OR OKNITHODELPHIA The principal distinguishing characteristics of this group have been already given (p. 371). They apply not only to the subclass, but of course equally to the one order MOXOTEEMATA, in which the few known members of the group are commonly associated. In addition to the more important characters enumerated above, the following, 1 &quot; Notice of Jurassic Mammals representing two New Orders,&quot; American Journal of Science, xx., September 1880. Aconints of further discoveries of forms allied to Plagiaulax, some surviving even to the earliest Tertiary period, are given by Cope in the American Naturalist for November 1881 and May 1882. which are common to all existing species, may be men tioned. The dorso-thoracic vertebrae are nineteen in number, and have no terminal epiphyses to their bodies. The transverse processes of the cervical vertebra are of auto genous formation, and remain suturally connected with the remainder of the vertebra until the animal is full-grown. Though in this respect they present an approximation to the Sauropsida (Reptiles and Birds), they differ from that group, inasmuch as there is not a gradual transition from these autogenous transverse processes of the neck (or cervical ribs, as they may be considered) into the thoracic ribs, for in the seventh vertebra the costal element is much smaller than in the other, indicative of a very marked separation of neck from thorax, not seen in the Sauropsida. The upper ends of the ribs are attached to the sides of the bodies of the dorsal vertebrae only, and not to the transverse processes. The sternal ribs are well ossified, and there are distinct partly ossified intermediate ribs. The cerebral cavity, unlike that of the lower Marsupials or the Reptiles, with which they have so many structural affinities, is large and hemispherical, flattened below and arched above, and about as broad as long. The cribriform plate of the ethmoid is nearly horizontal. The cranial walls are very thin, and smoothly rounded externally, and the sutures become completely obliterated in adult skulls, as in Birds. The broad occipital region slopes upwards and forwards, and the face is produced into a long and depressed rostrum. . The bony palate is prolonged backwards, so that the posterior nares are nearly on a level with the glenoid fossa. The mandible is without distinct ascending ramus ; the coronoid process and angle are rudimentary, and the two halves are loosely connected at. the symphysis. The fibula has a broad, flattened process, projecting upwards from its upper extremity above the articulation, like an olecranon. In the male there is an additional, flat, curved ossicle on the articulating chiefly to the tibia, which supports in the adult a sharp-pointed perforated horny spur, with which is con- i nected the duct of a gland situated beneath the skin of the ! back of the thigh, the function of which is not yet clearly ! understood. (A rudimentary spur is found in the young i female Ornithorhynchiis, but this disappears when the simple ; the alimentary canal has no ileo-caecal valve, or marked distinction between large and small intestine, but has a small, slender vermiform caecum with glandular walls. The liver is divided into the usual number of lobes characteristic of the Mammalia, and is provided with a gall bladder. Although agreeing in so many important characters, the existing members of the group evidently represent two very diverging branches, perhaps as far removed as are the members of some of the accepted orders of the Eutheria. It would, however, be encumbering zoological science with new names to give them any other than the ordinarily known family designations of Ornithorhynchidse and Echidnidse. Family ORNITHORHYXCIIIDJE. One genus, Ornithorhynchus, Bhmienbaeb, 1800. 2 Cerebral hemi spheres smooth. Premaxillfc and mandible expanded anteriorly and supporting a horny beak something like that of a duck, bordered by a naked and very sensitive membranous expansion. The place of teeth supplied functionally by horny structures, elongated, narrow, and sharp-edged along the anterior part of the sides of the mouth, and broad, flat-topped or molariform behind. Legs short, fitted for swimming; feet webbed, each with five well-developed toes armed with large claws, and beyond which in the fore feet the interdigital membrane is extended. Vertebrae : C 7, D 17, L 2, S 2, 2 The name Platypus, bestowed by Shaw in 1799, was preoccupied by a genus of Coleoptera. XV. 48
 * hinder and tibial side of the plantar aspect of the tarsus,
 * animal becomes adult.) The stomach is subglobular,