864 DR C. F. SONNTAG ON THE MYOLOGY 



but the fusion is best marked in Phascolarctos. Fibres i-un to 

 the pads of vibrissse, but none are continuous with the well- 

 developed muscles in the labial labrets in Phascoloniys. There is 

 no essential point of difference between the platysma in these 

 forms and the Phalangeridse. 



Masseter : — The general form is similar in Phascolomys (text- fig. 

 31 A) and Phascolarctos^ ; in the former the anterior border is 

 thick and muscular, but it is thin and tendinous in the latter. 

 In Phalanger (text-fig. 31 B) and Pseudochirus (text-fig. 31 C) the 

 form differs from the above : in the former the surface is 

 subdivided by sulci into three parts, but it is undivided in the 

 latter. 



Pterygoids : — Macalister (5) observed that the external ptery- 

 goid is small and fused with the inteivnal pterygoid in Phascolarctos, 

 but I fovind it separated and well developed in all forms. 



Digastric (text-figs. 32-34 t) : — In Phascolarctos it is mono- 

 gastric and fused with the mylo-hyoid, genio-hyoid, and omo- 

 hyoid ; and it is inserted into the mandible. I have already 

 shown that it enters into a sheet with mylo-hyoid and sterno- 

 hyoid (10), and I believe that the lateral fibi-es of the sheet 

 correspond to the digastric. Young (14) described two bellies 

 and a tevidinous inscription. In Phascolomys the broad anterior 

 bellies conceal the mylo-hyoid, and strong intermediate tendons 

 unite them to the posterior bellies. In Phalanger the broad 

 anterior bftllies do not completely cover the mylo-hyoid, and the 

 posterior bellies are fusiform. In Pseudochirus the monogastric 

 muscles conceal the greater part of the mylo-hyoid. In no species 

 has the digastric any connection to the hyoid bone. There is, 

 therefore, a close resemblance between the digastric muscles in 

 Phascolarctos and Pseudochirus. 



Mylo-hyoid (text-figs. 32-34, M-H.M) : — In Phascolomys and 

 the Phalangers the mylo-hyoid runs from the mandible to the 

 hyoid bone. But in Phascolarctos it does not get any attach- 

 ment to the hyoid. In my specimen it is fused with the 

 sternohyoid ; and thei-e is, in reality, a sterno-mandibularis. 

 Young (14) does not mention fusion of mylo-hyoid and sterno- 

 hyoid, but notes that the mylo-hyoid has no attachment to the 

 hyoid bone. 



Omo-hyoid (text-figs. 32-34, 0-H.M) : — In Phascolonnys and the 

 Phalangers it is attached to the hyoid bone, but no central tendon 

 is present. In Phascolarctos, however, it avoids the hyoid bone 

 and passes forwards to enter the tongue, wherein it forms a 

 lingualis. 



The ansa hypoglossi, which supplies the omo-hyoid and pre- 

 tracheal muscles, is derived from the cervical plexus alone in 

 Phascolarctos, Phascolomys, and Pseudochirus. In Phalanger I 

 observed the nerve receiving connections from the first three 

 cervical and the hypoglossal nerves (text-fig, 31 D), 



* P. Z. S. 1921, p. 548, text-fig. 53. f ■^**'^-> P- 549, test-fig. 54. 



