254 MR, F. G. PARSONS ON THE [Mar, 20, 



levator labii superioris of human anatomy. The anterior belly of 

 the occipito-frontalis may be made out as a thin layer of muscle 

 running upwards from the upper border of the orbicularis palpe- 

 brarum ; it is quite distinct in Hijstrix. The other facial muscles, 

 including those of the lower lip, are indistinguishable from the 

 facial panniculus. (See fig. 10.) 



Pterygoid Muscles.— The external pterygoid is small and rises 

 from the very feebly marked external pterygoid plate or rather 

 ridge ; it is inserted into the inner side of the neck of the condyle 

 and the bone just below. The internal pterygoid rises from the 

 outer side of the internal pterygoid plate or pterygoid bone, and has 

 the usual human insertion. In Sphingurus the internal muscle 

 consists of two distinct layers ; with this exception the above 

 description applies to all the animals examined. 



Digastric. — The digastric is attached posteriorly to the front of 

 the paroccipital process, ^^■hile anteriorly it is inserted into the 

 inner surface of the mandible. There are two absolutely distinct 

 types of the muscle. In the Hystricomorpha the two bellies are 

 not separated by a real tendon as in Man, although there is a 

 slight constriction of the muscle, and a thin layer of tendon on the 

 surface especially below ; the attachment to the hyoid bone is very 

 feeble, and the two anterior bellies are separated from one another 

 by a distinct interval in which the mylo-hyoid is exposed. The 

 anterior attachment is a considerable distance from the symphysis. 



In the ChinchilUdse the attachment to the hyoid bone is well 

 marked. 



In the Hystricidse the above description applies in Hystrioc, 

 but in Spliingiirus the posterior belly ends in a tendon which is 

 firmly inserted into the hyoid bone ; from the anterior or upper 

 side of this tendon the anterior belly runs to its attachment on the 

 mandible. 



The sciuromorphine type of digastric is well described by 

 Kunstler^ in the Marmot, The anterior and posterior bellies are 

 separated by a distinct tendon ^^•hich is firmly attached to the 

 hyoid bone. From the inner surfaces of the tendons of the two 

 sides fibrous bands run inwards to meet one another, forming a 

 tendinous arch with its convexity in front. Erom the front of 

 this arch the two anterior bellies spring : they are in contact in the 

 middle line nearly as far as the symphysis, Avhere they separate a 

 little to expose the transverse mandibular muscle. 



The tendinous arch gives attachment to some of the fibres of the 

 mylo-hyoid as well as the anterior bellies of the digastric. This 

 arrangement is common to all the Sciuroraorpha {Sciurus, Pteromys, 

 Xerus, Spermophilus, Arctomys, Castor) as well as the Dipodidae 

 (Dipus agyptius, D. Ixirtipes, Alectaga inclica). 



Transverse Mandibular Muscle. — This muscle is accurately 

 described by Kunstler in the Marmot ; it was present in all the 

 Eodents with a movable symphysis menti examined. I have not 



> Annales des Sciences naturelles, s^r. 7. t. iv. p. 1.50. 



