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Show 1896.] MYOLOGY OF RODENTS. 191 3. The absence of the transverse mandibular muscle. 4. The frequent origin of the acromio-trachelian (levator claviculae) from the basioccipital. 5. The usual presence of only one head of the biceps cubiti. 6. The occasional presence of the splenius colli in the Hystricomorpha and its constant presence in the Lagomorpha. 7. The basioccipital origin of the scalenus anticus when that muscle is present. The Sciuromorpha resemble the Lagomorpha and differ from the other two suborders in the following points:- 1. The rotator humeri portion of the coraco-brachialis is always present. 2. The supracondylar slip of the semimembranosus rises from the tuber ischii in both, but while it is closely connected to the adductor mass in the Sciuromorpha, it is separate from it and adherent to the rest of the semimembranosus in the Lagomorpha. On looking through these lists one cannot help being struck by the frequency with which certain muscles, such as the omo-byoid, the scapulo-clavicularis, the acromio-trachelian, the scalenus anticus, the splenius colli, the trachelo-mastoid, and the rectus abdominis, occur again and again. It is chiefly by various combinations of these muscles aided by a few others, such as the transverse-mandibular, masseter, digastric, biceps, coraco-brachialis, &c, that the affinities between animals belonging to the same group are marked ; and it seems to m e that one would be justified in saying that, in Rodents at all events, the muscles of the trunk and neck are the most valuable for classificatory purposes. It may be urged that all these muscles are liable to individual variation; and this of course is probably true, though I a m inclined to think that individual variations are far less frequent in Rodents than in M a n ; still if five or six of these muscles are taken, the risk of more than one being abnormal must be very slight indeed. As an instance of this the case of the Jerboa might be cited. In all the Hystricomorpha examined a scapulo-clavicularis had been found, but in the Jerboa it was absent. At that time I looked upon the animal, with Dobson, as hystricomorphine, and I regarded the absence of the muscle as an individual variation; as the dissection proceeded I found other points which were different to anything seen in the Hystricomorpha; later on I was lucky enough to get two more Jerboas of different species to the first one, and in both of these the scapulo-clavicularis was wanting also. Without the confirmatory testimony of the other muscles, the absence of the scapulo-clavicularis would probably have been passed over as of little importance. If the long flexors of the foot are considered, it will be found that they are not so rehable as the muscle just quoted. Dobson'says that these long flexors unite in the Hystricomorpha but not in the other suborders ; I have found, however, three animals-Rhizomys, Heteromys, and Pterqmys-which have no |