44 THE VOYAGE OF H.M.S. CHALLENGER. 



described by Dr. Young in the Koala l ) is very evident. The upper thick fleshy mass is 

 fixed on the one hand to the posterior surface of the tibia and on the other to the antero- 

 internal border of the fibula. A few very oblique and superficial fibres arising from the 

 external lateral ligament of the knee-joint evidently represent the popliteus proper, the 

 rest of the muscle being the pronator tibiae of Humphry. The lower thickening of the 

 muscle is fixed to the antero -external surface of the tibia. 



The movements of the fibula upon the tibia at the tibio-fibular joints cannot in any 

 respect be compared to those of pronation and supination in the forearm. Young, in his 

 admirable paper upon The so-called Movements of Pronation and Supination in the 

 hind-limb of certain Marsupials, 1 has already pointed out that the " tibio-fibular 

 articulations admit of movements in the antero-posterior directions only ; these move- 

 ments are simply those of gliding and coaptation." He also shows how the attachment 

 of the interosseous muscle on different planes affects this movement ; the upper portion 

 of the muscle producing " a backward and inward movement of the upper end of the 

 fibula," whilst the lower fibres by their contraction draw the lower end of the bone 

 inwards and forwards, or in other words, when the whole muscle contracts, the fibula is 

 " thrown obliquely across the tibia " and approximated to it. From a study of the 

 pronator tibiae muscle, and the tibio-fibular articulations in the discus, I can verify Dr. 

 Young's results. 



In the Thylacine the movement of the fibula upon the tibia is extremely limited, but 

 of precisely the same character as in the Cuscus. The pronator tibia? muscle is, however, 

 very well developed, but owing to the close apposition of the bones, the fibres are 

 exceedingly short. It shows the same thickening in its upper and lower portions, and 

 below it is chiefly seen from the anterior aspect of the limb, owing to the tibial attach- 

 ment of the fibres being so far forward upon the bone. 



As in the Koala, the tibia and fibula in the Cuscus are separated at the lower tibio- 

 fibular articulation by a fibro-cartilage, which juts upwards between them from the 

 ankle-joint. In Thylacinus, however, the lower ends of the bones are in direct apposition, 

 and both surfaces are coated with encrustins: cartilage. 



Anterior and Outer Aspects of the Leg. 



In the Cuscus there are three well-defined muscles upon the anterior aspect of the leg, 

 viz., the tibialis anticus, the extensor longus hallucis, and the extensor longus digitorum. 

 These muscles occupy the same relative position as in man. In Thylacinus the extensor 

 longus hallucis is absent. 



Tibialis anticus (PI. V. fig. 3,f). — This is a very powerful muscle in both animals ; it 



1 Jour. Anat. and Phys., vol. xv. p. 393. 



