RODENTIA (Glires, Linn.) (Fr.l?ongeurs), —An important order of mammiferous Verte brate, distinguishable by the remarkable struc ture of their incisor teeth, which are adapted to perform the office of chisels by cutting and gnawing away the hard vegetable substances, -which form their principal food. The animals of this order, indeed, appear to be specially ap pointed to devour the hardest substances, ge nerally living upon the wood and bark of trees, as well as upon nuts and other shelled fruits. The incisor teeth, which characterize the ani mals of this order, are situated in both jaws, and are separated from the molar by a considerable space, so that they are ill-adapted to seize living prey, or to devour flesh, notwithstanding that certain genera of rodents exhibit decidedly carnivorous propensities. These incisors, also called dentes scalprarii, are only provided with enamel upon their ante' ior surface, so that the posterior portion of the tooth being worn away more rapidly than the an terior, these teeth always present a chisel-like edge. The lower jaw is articulated to the cranium by a longitudinal condyle, in such a manner that it has no horizonal motion ex cept from before, backward, and vice versi :1; a movement adapted to effect the act of Those genera in which these layers of enamel are simple plates, and which have the crowns of their molar teeth very flat, are more particularly frugivorous ; those in which the eminences of these teeth are divided into blunt tubercles, are omnivorous ; whilst a small number of genera, which possess pointed mo lars, will attack other animals, and in some of their habits approximate the Carnivore.
This order comprises the following genera :— Sciurus (Squirrel). Pteromys (Flying Squirrel) (fig. 247.). Cheiromys (Aye-Aye). Arctomys (Marmot). Rlyoxus (Dormouse). Echimys. Hydronlys. Capromys. Mus (Rat). Gerbillus. Meriones. Cricetus (Hamster). Arvicola (Vole). Fiber (Musk Rat). Geo rychus (Lemming). Otomys. Dipus (Jer boa) (fig. 248.). Pcephagomys. Helamys. Spalax (Rat Mole) (fig. 249.) Bathiergus
(Cape Mole) (fig. 250.). Geomys. Diplo stoma. Castor (Beaver). Myopotamus (Coui). Hystrix (Porcupine). Lepus Mare). Lego mys (Rat Hare). Hydrochcerus (Cap) bare). Rhyzomys. Cavia (Guinea Pig). Dasyprocta (Agouti). Ccelogenys (Pace). Chinchilla.
Bones of the cranium. —The bones of the cranium in the Rodentia present several pecu mina are united into one, in front of which the sphenoid forms a single vertical lamella, liarities in their arrangement, which it will .be necessary to notice.
an evident approximation to what is found in birds.
In the hare*, the anterior sphenoid is very remarkable, inasmuch as the two optic fora * Cuvier, Anatomic compar4e, last edition.
The os frontis presents a strong supra orbital crest, which is deeply notched both before and behind. It advances on each side by a long process, between the ascending point of the inter-maxillary bone and that portion of the maxillary which forms the cheek : the parietals remain for some time distinct from each other, and from the inter-parietal ; which latter, in the rabbit, is small, and resembles an ellipse placed transversely : in the hare this last bone can only be detected in very young specimens, when it is found to con sist of two small pieces, which are separated by a prominent angle of the occipital. The petrous portion of the temporal bone occu pies a large triangular space in the occipital region of the skull. The mastoid process is entirely formed by the occipital bone; but the os petrosum furnishes a parallel process, which embraces the temporal externally, and at an early period it becomes united therewith. The tympanic portion of the temporal is con siderably arched, but is far from reaching the pterygoid processes. The temporal of the posterior sphenoid do not mount up very high, and do not reach the frontal, from which they are separated by the anterior sphenoid and by the temporal, still less do they ap proximate the parietal bones, which do not descend so low as the temporal.