258 MK. i\ G. PARSONS ON THE [Mar. 20, 



Castor it is entirely covered by the trapezius, and is inserted 

 into tlie upper border of the acromial process and the outer part 

 of the spine of the scapula. 



The above observations show that the levator claviculae is a 

 muscle of very little use for classificatory purposes among the 

 Hystricomorpha ; in this group it seems to vary even in different 

 individuals of the same species ; it is possible that it is in process 

 of shifting its attachment from the basioccipital to the front of the 

 atlas. The number of observations, however, are not sufficient 

 for generalization. 



Rectus Ca^ntis Anticus Major and Minor. — The rectus capitis 

 anticus major rises in the Hystricomorpha fi-om the transverse 

 processes of two or three cervical vertebrse below the atlas, and is 

 inserted into the basioccipital bone in front and internal to the 

 scalenus anticus ; except in the Caviidse it is difficult to separate 

 from the longus colli. In the Sciuromorpha the muscle usually 

 rises from a greater number of transverse processes. The rectus 

 capitis anticus minor and lateralis have the human attachments ; 

 the latter is large and closely connected to the superior oblique. 



Longus Colli. — The longus colli closely resembles the same muscle 

 in Man ; it consists of two oblique and one straight part. The 

 posterior obUque part rises fi'om the bodies of the anterior three 

 or four thoracic vertebrae, and is inserted into the transverse pro- 

 cesses of the posterior cervical vertebrae. The anterior oblique 

 portion runs from the insertion of the last part to the longus colli 

 tubercle on the ventral arch of the atlas. The straight part runs 

 from the bodies of the anterior thoracic vertebrae to those of the 

 anterior cervical. In Castor it extends a long way into the thorax. 



Scalemis Anticus. — As there is a good deal of difficulty in identi- 

 fying the scalene muscles of Eodents with the three scalenes of 

 human anatomy, I have given the name of scalenus anticus only to 

 a muscle inserted into the first rib between the subclavian artery 

 and vein. This muscle when present rises by a tendon from the 

 basioccipital in front and internal to the levator claviculae ; in 

 Ccelogenys it also derives a few fibres from one or two cervical 

 transverse processes. It is absent in the Hystricidae {Hystrix, 

 Sphingurus) and in all the Sciuromorpha, but present in the other 

 animals examined. 



Scalenus Meclius and Posticus. — These two muscles are most 

 conveniently described together, as it is often impossible to say 

 where one ends and the other begins. 



In Aulacodus, which is a good type of the arrangement in the 

 Octodontidae, one muscle, which I take to represent the scalenus 

 medius, rises from the transverse processes of the first four cervical 

 vertebrae and is inserted into the sides of the 4th and 5th ribs 

 between the serrations of the serratus magtius. Another muscle, 

 probably the scalenus posticus, rises from the posterior three 

 cervical transverse processes and is inserted into the first and 

 second ribs. 



In Chinchilla the arrangement is almost identical. 



