AFFINITIES OF RODENTS. 43 



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 con- 

 tinuously 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 

 simpliridentati, 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 intra- 

 uterine 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, 



