AND MYOLOGY OF THE MARSUPIALIA. 853 



contact with the sterno-mastoid. In all other Marsupials the 

 posterior triangle is evident, and its most prominent structure 

 is the external jugular vein. The descending cutaneous nerves 

 emerge from between the muscles in Dendrolagus (text-fig. 74). 



The Omo-hyoid is present in all Marsupials, and is strap-like 

 or fusiform. A central tendon is present in Macropus rvfas (19) 

 and a slight one exists in Petrogala xaniJiopus (13). In a pouch 

 specimen of Macrojpus glganieus (text-fig. 71a) it crosses the 

 lateral thyroid lobe which projects beyond the pretracheal muscles, 

 and Osgood (9) figures it crossing the anterior lymphatic gland in 

 Coinolestes obscurios. 



The Sterno-hyoids form a uniform strip from origin to insertion 

 in all Marsupials except a male pouch specimen of Macropits 

 giganteics (text-fig. 71c), in which they diverge at their hyoid 

 attachments, and the larynx protrudes between them. They are 

 easily separated, as a rule, from the subjacent sterno-thyroids, 

 but that cannot be done in Perameles obesula. This would show 

 that these muscles arise from a single sheet by splitting. 



In my paper on the Koala and Vulpine Phalanger (15) 

 I showed that the mylo-hyoid, digastric, and sterno-hyoid muscles 

 of the former form a thin sheet playing over, but in no way con- 

 nected to the hyoid bone ; but I did not observe this condition in 

 any other Marsupial ; it may also be an abnormality in the 

 animal examined by me. And in no other were there so many 

 small muscle bundles running between the digastrics and sur- 

 I'ounding muscles. The fusion of the digastric and mjdo-hyoid 

 supports Gegenbaur's view that these muscles were differentiated 

 by splitting. 



The Digastric has a small central tendon in Macropus, but 

 none is present in Dendrolagus, Perameles, and Metachirtcs. In 

 Peirogale xanthopus (13) there are small fibrous areas, but no 

 true central tendon. The mandibular insertion is usually about 

 half the length of the bone between the angle and the sym- 

 physis, and is usually muscular, but Osgood (9) gives a more 

 extensive attachment from angle to symphysis in Cmnolestes 

 obscurus ; and the anterior part is more apotieurotic. 



The Hyoglossus was absent in my specimen of Phascolarctos 

 cinereus, and is not described in Macalister's paper on the same 

 animal. It is present in all other Marsupials. Its absence in 

 the Koala is probably due to the long interval between the tongue 

 and the hyoid bone, which has few attached muscles. 



Carlsson (2) desci-ibed the myology of Dendrolagus doriamos in 

 detail, and grouped the muscles in three sets : — 



1. Those resembling tlie muscles of the terrestrial Macro- 

 podidfB and differing from those of the Phalangeridse. 



2. Muscles resembling those of the Phalangeridae and differing 

 from those of the terrestrial Macropodidae. 



3. Conditions which differ from those of the terrestrial Macro- 

 podidae and PhalangeridsR, and are secondary adaptations for an 

 arboreal life. 



