3 
1896.] MYOLOGY OF RODENTS. 191 
3. The absence of the transverse mandibular muscle. 
4, The frequent origin of the acromio-trachelian (levator 
clavicule) 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 Hystrico- 
morpha 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-hyoid, 
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 com- 
binations 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 me 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 am inclined to think 
that individual variations are far less frequent in Rodents than in 
Man; 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 reliable as the muscle just 
quoted. Dobson says that these long flexors unite in the Hystri- 
comorpha but not in the other suborders ; I have found, however, 
three animals—Rhizomys, Heteromys, and Pteromys—which have no 
