164 MR. F, G. PARSONS ON THE {Jan. 14, 
animals approach more or less closely to the Hystricomorphine 
arrangement. 
In Myoawus the type is essentially Sciuromorphine, 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 Gerbillus (where, however, the posterior bellies rise from 
the bulla tympani), in Cricetomys, in the Murine (Mus raitus, 
M. decumanus, and M. barbarus), and in Rhizomys among the 
Spalacide. In Cricetus, Microtus, Myodes, and Hydromys (Windle) 
the central tendon is reduced to a mere tendinous intersection as 
in the Hystricomorpha, but the two anterior bellies are still in 
close contact. In Bathyergus and Georychus among the Spalacide 
a similar arrangement is tound, but, as in all other cases, the 
double nerve-supply of the muscle is preserved. Heteromys 
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 Muride than in the 
other families. When the mylo-hyoid comes far enough forward 
the transverse mandibular is superficial to it. 
Mylo-hyoid.—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, the Murine (MV. rattus, decumanus, and barbarus), 
and in Heteromys 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 Sciuromorpha. 
Genio-hyo-glossus.—This has the usual attachments ; it is speci- 
ally large in Rhizomys. 
Styloid Muscles.—The stylo-hyoid has the usual Rodent arrange- 
ment in passing deep to the digastric. 
In Rhizomys these two muscles are closely blended, while in 
Georychus the blending seems to be more complete, for no distinct 
stylo-byoid could be made out. The stylo-glossus rises by tendon 
from the posterior part of the bulla; it is very well marked in 
Cricetomys, while in Georychus and Bathyergus 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. 
Sterno- and Cleido-mastoid.—In the Myomorpha these two 
muscles are perhaps not quite so distinctly separated one from 
another as in either of the other suborders. 
In Cricetus, Myoxus, Microtus, Myodes, Mus barbarus, M. rattus, 
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 line of the occipital bone, close to the 
