338 NATURAL SCIENCE. Nov., 



may be urged that the presence of the muscle in these subterranean 

 forms depends more on their mode of life tlian on their relationship. 

 Against this view I would urge the case of the mole-rat, Rhizomys, 

 which is also subterranean, yet which does not possess the muscle, 

 and also the fact that the muscle is found in all the Hystricomorpha, 

 including animals such as the tree-porcupines, the agoutis, and the 

 cavies. It is interesting to turn aside for a moment to speculate on 

 the methods by which this muscle might appear or disappear. Only 

 two occur to me : first, that it is a delamination from the subjacent 

 supraspinatus ; secondly, that it has been formed by the conversion 

 into muscle of the fascia over the supraspinatus, by the encroach- 

 ment of fibres from the subclavius. In favour of the latter, and 

 against the former, hypothesis are the facts that the muscle is often 

 continuous with the subclavius, and that it is supplied by the same 

 nerve and not by the suprascapular nerve, which supplies the supra- 

 spinatus. 



Both the muscles already selected as examples tend to show that 

 the myomorphine arrangement is more closely allied to the sciuro- 

 morphine than to the hystricomorphine and lagomorphine. It is not 

 difficult to find other examples of this. For instance, the small 

 transverse mandibular muscle, which unites the two halves of the 

 lower jaw close to the symphysis, is present in the Sciuromorpha and 

 Myomorpha, absent in the Hystricomorpha and Lagomorpha. 



The scapulo-clavicularis is an instance of a muscle which is not 

 found at all in the Sciuromorpha, is always present in the Hystrico- 

 morpha and Lagomorpha, and is very rarely seen in the Myomorpha. 

 I will next give instances of muscles which are present in the more 

 generalised squirrel group, and are gradually lost as we ascend to the 

 more specialised. The above-mentioned transverse mandibular 

 muscle is one instance of this ; another is the omo-hyoid, which is 

 always present in the Sciuromorpha and Myomorpha, but is absent in 

 certain families of the Hystricomorpha, such as the Chinchillidae, 

 Dasyproctidse, and Caviidae. In the Hystricidae it is absent in the 

 ground-porcupines Hystvix and Atherura, but present in the tree- 

 porcupines Sphinguvus and Evethizon. In the Lagomorpha the muscle 

 is absent in the hare and rabbit.' The presence of the omo-hyoid in 

 the tree-porcupines and its absence in the ground-porcupines may 

 certainly be regarded as an instance of change of musculature 

 accompanying change of habits, more especially as there is, so far as 

 I am aware, no arboreal rodent which does not possess an omo-hyoid. 

 My object, however, is not to prove that this never occurs, but rather 

 to show that, in spite of it, many muscles vary very constantly with 

 the relationships of the animals that possess them, and may be 

 advantageously considered in classification. 



. Another muscle on which I am inclined to lay a good deal of 



^I have, unfortunately, never had the opportunity of dissecting a Pica, the other 

 genus of this suborder, nor can 1 lind any account of its myology. 



