There can scarcely be any doubt that the Insectivora must be considered to be as ancient a form of mammalian life as the Rodentia; indisputable remains however of the order have not as yet been found lower than the Middle Eocene, whilst remains of Rodents have been identified in the lowest Eocene, the Cory-phodon beds of North America. This absence of Insectivora however must be ascribed to the present imperfection of the Geological record; and it should be added that a jaw of a small Mammal (Dromotherium) from the Trias of North Carolina, though supposed to have belonged to some Entomophagous Marsupial allied to Myrmecobius, is not definitely proved not to have belonged to a true Insectivora, and indeed that the likeness between these two sets of animals is a very strong argument for the antiquity of the Insectivora, as also for their inferiority when compared with the Rodentia, at least as regards the lowest members of each order.
This inferiority is manifested in the Geographical Distribution of the two orders. Though both alike are favoured as regards spreading over the world by the smallness of their size and their faculty of hibernation, and, in the case of certain Insectivora, of aestivation also, the Insectivora are obviously a 'failing' order, being, though represented in all the Zoo-geographical regions except the South American and Australian, still poor in numbers both of individuals and of species.
Many points, however, bear evidence to the antiquity of the Rodent type of Mammalian life, and to its alliance with still lower forms, such as the Marsupials and the Sauropsida, which is correlated with that antiquity. The imperfect ossification which leaves perforations or fenestrations in many cranial and facial bones, and allows sutures such as those of the basicranial bones and the symphysis of the jaw to remain unanchylosed, while in higher Mammals we find them continuously ossified; the retention both of bones such as the presphenoid and of processes such as the basipterygoid in a distinctness and independence which is lost in higher forms; the small size of the coronoid process and the tendency to inversion of the angle of the mandible, the occasional persistence of the vomer in two distinct moieties, and the constant imperfection of the orbital ring, are some of such points furnished by the skeleton.
Great as are the variations observable within the limits of the order Rodentia, all living Rodents agree in the following particulars: they have the homologues of the two central incisors in both jaws furnished with permanently growing pulps, and with chisel-shaped cutting edges; these teeth are so largely developed as to have caused all the other incisors to abort either wholly, as in the Glires simplicidentati, or all but wholly, as in Glires duplicidentati or Lagomorphi' and the canines to abort invariably, leaving thus a diastema between the incisive and the molar series of teeth. The symphysis of the lower jaw is never anchylosed perfectly. The greatly developed incisive bones always separate the maxillaries from the nasals, but are themselves separated by processes of the maxillaries from the lacrymals. The lacrymal canal opens inside the orbit. The orbital ring is never perfect. The maxillary bone always forms a part of the jugal arch together with the malar and the squamous. The omphalo-mesenteric vessels contribute importantly to the nourishment of the foetus during the whole of intrauterine life; the allantoid or true placenta is attached to the mesometrial part of the circumference of the uterus.
Till the discovery of the singularly aberrant Lophiomys Imhausii, the absence of the opposable hallux so commonly observable in Marsupials was supposed to be characteristic of all Rodents. This Rodent, in which the malars-and parietals extend over the temporal fossa, as is the case in Chelonia, but in no other known Mammal, is also the known Rodent which is pedimanous. See A. Milne Edwards, Nouv. Arch, du Muse'um, 1867, p. 114. The name 'Prensiculantia' or 'Pfotler' was suggested by Illiger, Prodromus, p. 81, 18115 after Buttmann, as a substitute for that of Glires upon the following grounds: - 'Nota: nomen hujus ordinis a celeb. Buttmann excogitatum est ut maniculorum instar manuum usus his Mam-malibus familiaris indicetur, aliis e motus instrumento desumtis notis toti ordini communibus simulque characteristicis, deficientibus.'
The differences between the Glires duplicidentati, s. Lagomorphi, represented by the Rabbits, the ordinary Hares, and the Tailless Hares, and all the other living Rodents, are so great, and those which separate the three other subdivisions of the order, the Myomorphi, represented by the Mice, the Sciuromorphi, represented by the Squirrels, and the Hystricomorphi, represented by the Porcupines and Cavies, are so much smaller as to make it convenient to divide the order into two main divisions or Suborders: viz. the Simplicidentati, comprehending the three sections just mentioned, and the Duplicidentati, comprehending the Hares and Rabbits. Of these last the following propositions may be made in contradistinction to the Simplicidentati.
They have, as adults, two small incisors placed behind the two large ones in the upper jaw, these two pairs of teeth representing the anterior and the posterior of the three pairs of early life. They have a larger number of teeth, f in the molar series, than any other Rodents. The incisors are surrounded by a perfect zone, if not of enamel in the adult, at least of enamel membrane in the developing tooth. The enamel of the incisors is not divisible into two layers. The incisive and the optic foramina are, severally, confluent, and the bony palate greatly reduced. The glenoid fossa and the articular condyle of the lower jaw are less specialized to antero-posterior movement than is the case in other Rodents. The coronoid process and the sockets for the incisors in the same bone are also less specialized. Ossification is less perfect, as for example in the facial part of the maxillary and in the basicranial bones. They have a canalis caroticus in the tympanic, but no true alisphenoid canal. The fibula is anchylosed to the tibia, but articulates with the os calcis. The placenta is not disc-shaped, but consists of two or more lobes sessile on the chorion and clamped together by a saddle-shaped decidua serotina.