Bones of the carpus.— Generally, in the Rodentia, the os magnum is divided into two, as it is in the monkeys ; in the porcupine this is not the case, but there is a supernumerary bone interposed between the os pisiforme and the metacarpal bone of the fifth finger con nected with the os unciforme.
The hare and some other Rodents have one carpal bone more than the human subject ; it is situated between the scaphoid, the trape zium, and the os magnum, of which last it appears to be a dismemberment; but the beaver, the marmot, the squirrel, the rats, and the agoutis have, like the Carnivora, a single bone representing the scaphoid.
The supernumerary bone is as large as the ordinary pisiform, and often much larger. Sometimes, as in the jerboa and the marmot, there are two supernumerary bones, so that, on each side of the wrist, there is a bone of equal size out of rank.
In the capybara the scaphoid and the semi lunar bone are united without any supernu merary ossicle; a small one, however, exists in the Guinea-pig. The paca, the agouti, and the capybara have the os magnum divided ; these three animals possess, as the rudiment of the thumb, a small bone situated upon the trapezoid, with which it is articulated.
In the marnzot and the agouti this rudiment is composed of three ossicles ; and there is, moreover, an internal supernumerary bone.
in the order Rodentia the structure of the thumb differs in different genera ; there is a complete but short thumb in hares, beavers, and jerboas ; an incomplete thumb, consisting of only two phalanges, in squirrels, rats, por cupines, pacas, and agoutis ; and a thumb, represented by only a single ossicle, in the capybara, the Guinea-pig, the marmot, &c.
In connection with the fore-arm it may be observed, that the rabbit has only one pronator of the urist, corresponding to the pronator teres ; a circumstance easily accounted for by the very small degree of motion permitted between the bones of the fore-arm ; in most other Roclentia, however, both the pronators are present.
The terminal phalanges of the fingers are generally very slender, elongated, almost straight, and pointed, except in the capybara, which has its last phalanges of a triangular shape, and enclosed in strong horny hoofs.
In those Rodentia which have a complete clavicle, the muscles of the shoulder resemble very nearly in their disposition those of the human subject. The humerus resembles
that of the Carnivora in its mode of articulation with the fore-arm ; but in those genera that are without clavicles, the articu lation of the elbow joint resembles inore nearly what is met with in herbivorous quad rupeds, being a simple hinge joint. The hu merus of the beaver is much expanded at its ulnar extremity, and the deltoid crest is pro In the Rodentia the general form and posi tion of the pelvis is nearly similar to what exists in the Carni vora.
The femur is in the beaver very broad, flattened from before to behind, and exhibits along its outer surface a sharp crest, which represents the linea aspera, and which is pro longed towards its middle into an apophysis, which has been named the third trochanter. This third trochanter is also met with ip other rodents, as, for example, in the musk rat ; in the hares it is placed so high up, that it ap pears to be a derivation from the great trochan longed inferiorly into a prominent point ; a circumstance which is likewise observable in the rat, the water vole, the ondatra, and in many other genera.
In the hares, the porcupine, the paca, and the agouti, the humerus near the elbow joint is completely perforated.
ter ; in other genera, as in the water vole, the rats, the squirrels, and the marmots, it is a simple crest, or ex ternal linea aspera ; in all the Rodentia the great tro chanter is very prominent, and the neck of its posterior aspect there is likewise a nent crest. It results from this structure, the thigh bone considerably narrower than its head.
The Rodentia have the fibula situated quite behind the tibia ; in rats, voles, jerboas, the beaver, the helamys, and the rabbit, it becomes consolidated with the tibia towards the lower third of its length, a wide triangular space being left between the two bones at the upper part of the leg ; the anterior crista of the tibia in all the above genera is remarkably prominent, as is the internal edge ; and upon that, viewed from behind, the tibia exhibits, in the upper half of its length, two deep fossm for the attachment of the tibialis posticus and the flexor longus pollicis. This struc ture is more particularly remarkable in the ondatra.