XX. I 



MARSUPIAL SKELETON 



559 



The general plan of the muscles and backbone is essentially that 

 found in placentals and has been greatly changed from the reptilian 

 or monotreme condition. There are no cervical ribs. The thoracic 

 region consists of about 13 rib-bearing vertebrae, as in placentals, and 

 there are about the usual 7 lumbars. The pectoral girdle shows no 

 interclavicle, but the clavicle remains large. The coracoid is reduced, 



Fig. 333 Fig. 334 F IG . 335 . p OU ch young of 



Figs. 333-4 Hind foot of A, Trichosurus, Dasyurus about 10 days old. 



and B, Macropas, showing elongation of (After Hill and Osman Hill.) 



the toes of the latter and reduction of the 

 1st, 2nd, and 3rd toes. (From Zittel, 

 Text-book of Palaeontology, Macmillan, 

 after Dollo.) 



as in placentals, and the scapula enlarged and provided with a spine. 

 In fact all the developments of the dorsal region of the girdle that are 

 typical of the mammalian method of locomotion have taken place. In 

 the pelvic girdle there are epipubic bones, reminiscent of those of 

 monotremes; they take the special stresses produced by the abdominal 

 muscles and pouch, but are reduced in the fully terrestrial and quad- 

 rupedal Tasmanian wolf. The hands usually carry five digits, armed 

 with claws, but the number of toes is often reduced and they may 

 bear hoof-like structures (Figs. 333—4). 



The Miillerian ducts are paired and differentiated into upper 

 'uterine' and lower 'vaginal' portions. In many species the latter are 

 provided with median diverticula, the two meeting in the mid-line 

 as a median vagina (Fig. 336). This ends blindly until the young are 

 about to be born, when an opening is formed through the tissues — the 



