1896.] il^^OLOQY 01' ilODENl>3. IPl 



3. The absence of the transverse mandibular muscle. 



4. The frequent origin of the aoroinio-trachelian (levator 

 claviculse) 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-bracbialis 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-liyoid, 

 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&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 liodents 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 Eodents 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 aU the 

 Hystricoraorpha 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 Pleromys — which have no 



