164 MB. F. G. PAKSONS ON THH [Jan. 14, 



animals approach more or less closely to the Hystricomorphine 

 arrangement. 



In Myoxus the type is essentially Sciuroraorphine, there is a 

 distinct central tendon and a tendinous arcade connecting the 

 anterior bellies of opposite sides ; the two anterior bellies, more- 

 over, are in contact in the middle line. The same arrangement is 

 found in Oerhillus (where, however, the posterior bellies rise from 

 the bulla tympaui), in Oricetomys, in the Murinaj (il/us ratius, 

 M. decumanus, and M. harharvs), and in Rhizomys among the 

 Spalacidje. In Cricetus, Microtus, Myodes, emA Hydromys (Windle) 

 the central tendon is reduced to a mere tendinous intersection as 

 in the H)'stricomorpha, but the two anterior bellies are still in 

 close contact. In Bathyergus and Qeoryclms among the Spalacidaj 

 a similar arrangement is found, but, as in all other cases, the 

 double nerve-supply of the muscle is preserved. IleUromys 

 approaches most nearly to the Hystricomorphine type, since the 

 two anterior bellies are not in contact and there is no tendinous 

 arcade ; there is, however, a distinct constriction and tendon 

 between the anterior and posterior bellies. 



Transverse Mandibular Muscle. — This muscle is present in all 

 cases ; it is perhaps better developed in the Muridoa than in the 

 other families. When the mylo-hyoid comes far enough forward 

 the transverse mandibular is superficial to it. 



Mylo-Jiyoid.—The mylo-hyoid is attached posteriorly to the 

 tendinous arcade connecting the digastrics and to the hyoid bone. 

 Anteriorly the two muscles usually form a V-shaped border, with 

 the aperture of the V forward, and do not reach the symphysis. In 

 Myoxus, Microtus, tlie Murinae {M. rattus, decumanus, and barlarus), 

 and in Heieromys the muscle extends farther forward than in the 

 rest and is then deep to the transverse mandibular. 



Genio-hyoid. — The two muscles of opposite sides tend to coalesce 

 posteriorly as in the Sciuromorplia. 



Oenio-hyo-c/lossus. — This has the usual attachments ; it is speci- 

 ally large in Rhizomys. 



Styloid Muscles. — The stylo-hyoid has the usual Eodent arrange- 

 ment in passing deep to the digastric. 



In Rhizomys these two muscles are closely blended, while in 

 Oeorychus the blending seems to be more complete, for no distinct 

 stylo-hyoid could be made out. The stylo-glossus rises by tendon 

 from the posterior part of the bulla ; it is very well marked in 

 Oricetomys, while in Oeorychus and Bathyeryus it rises by tendon 

 from the stylo-hyal element of the hyoid arch. As in other rodents 

 the stylo-pharyngeus was not seen as a distinct muscle. 



Stemo- and Oleido-masioid. — In the Myomorpha these two 

 muscles are perhaps not quite so distinctly separated one from 

 another as in either of the other suborders. 



In Oncetus, Myoxus, Microtus, Myodes, Mus barbarus, M. ratttts, 

 and Heteromys the cleido-mastoid rises from the inner part of the 

 bony clavicle under cover of the clavicular insertion of the trapezius ; 

 it is inserted into the curved Hue of the occipital bone, close to the 



