REPORT ON THE MARSUPIALIA. 133 



Beaver alone is it a tolerably well developed muscle. On reflection, the reason of this 

 weakness of the marginal abductors in aquatic feet becomes apparent. In the backward 

 stroke of the pes the water, the resisting medium against which it strikes, is sufficient to 

 cause the full expansion of the web. The flexorcs breves guard against over extension of 

 the digits, and thus the long flexor muscle is free to act upon the ankle joint. Dr. 

 Murie describes in the foot of the Sea-lion ' (Otaria jubata) two short muscles which lie 

 obliquely upon the under surface of the tarsus. They both arise from the scaphoid, and 

 they are respectively inserted into the internal cuneiform, and into the base of the first 

 metatarsal bone. He considers that they represent the adductor and flexor brevis 

 hallucis, but as he figures a very distinct adductor for this toe, and also the fibular 

 head of a flexor brevis hallucis, I think (as already explained) that it is more probable 

 that they are homologous with the absent abductor hallucis and tibial head of the flexor 

 brevis hallucis. But what has caused those muscles to assume these highly anomalous 

 attachments ? Dr. Murie states that " both are flexors " ; one of the internal cuneiform 

 upon the scaphoid and the other of the first metatarsal bone. In this manner therefore 

 they support the tarsus and enable it to bear the strain which must be communicated to 

 it by the backward stroke of the expanded digits through the water. In the Walrus 2 

 Dr. Murie found corresponding muscles, and in this animal, as we have seen, a special part 

 of the abducting apparatus of the minimus is set aside apparently for the same purpose. 

 This muscle stretches between the under surface of the os calcis and the base of the fifth 

 metatarsal bone. 



Let us now choose two terrestrial animals of similar habits, and which employ 

 their feet for similar purposes, viz., the Dog and the Thylacine. Both these animals use 

 their feet for progression, and it may be also occasionally for burrowing. In the latter 

 case they employ them to clear away the earth which has been detached by their fore- 

 paws. In both animals the feet are very similar in outward appearance, with the exception 

 that the Dog is digitigrade, whilst the Thylacine is semi-plantigrade. The intrinsic 

 muscles are disposed differently in each case. In the Thylacine the arrangement is 

 typical in every respect ; in the Dog the flexores breves and abductors are coalesced, and 

 this withstanding the slow, skulking character of the Thylacine as compared with the 

 highly active habits of the Dog. 



Arboreal Marsupials almost invariably exhibit the typical disposition of their 

 intrinsic muscles, e.g. the Phascogale, the Virginian Opossum, the Australian Opossum, 

 the Cuscus, &c. The Koala is an exception, inasmuch as the dorsal layer is poorly 

 represented. The arboreal Rodent, the Squirrel {Sciurus vulgaris) has, however, quite a 

 different arrangement. The adductors and flexores breves are well developed, but the 

 abductors are reduced to a single muscle, viz., the abductor minimi digiti. 



1 On the Anatomy of the Sea Lion, Trans. Zool. Soc, vol. vii. p. 579. 



2 On the Anatomy of the Walrus, Trans. Zool. Soc, vol. vii. p. 456. 



