REPORT ON THE MARSUPIALIA. 59 



origin from the base of the middle metatarsal bone, and crossing obliquely the metatarsal 

 of the annular digit, is inserted into the outer side of the base of the proximal phalanx 

 of this toe. The second slip (cP) lies in the fourth interosseous space, and springs from 

 the adjacent bases of the two metatarsal bones between which it lies, and, like the corre- 

 sponding slip of the third dorsal interosseous muscle, ends in a tendon which bifurcates 

 to gain a double insertion into the contiguous sides of the bases of the proximal phalanges 

 of the annularis and minimus. 



The third and fourth dorsal interossei, therefore, each consists of two slips, of which 

 one possesses the characteristic insertion of the muscle to which it belongs, and must in 

 consequence be looked upon as an abductor, whilst the other, by its peculiar double 

 insertion, must act as an approximator of the digits into which it is inserted. In the 

 Cuscus therefore, there is apparently a tendency to the formation of a fourth layer of 

 muscles, endowed with a new function, by the splitting of the dorsal interossei. In 

 the manus of the Thylacine a similar splitting of the dorsal interossei and the development 

 of approximating muscles has already been noted (p. 22). 



In the foot of the Cuscus an opponens minimi digiti is also to be found (op. 6 ). It is a 

 strong quadrate muscle, placed obliquely in the pes, which arises from the outer margin 

 of the plantar cartilage, and is inserted into the whole length of the fibular margin 

 of the metatarsal bone, and also by a few fibres into the outer aspect of the base of the 

 first phalanx of the minimus. 



Phalangista vulpina (Australian Opossum). 



The foot of this animal in external characters closely resembles that of the Cuscus. 

 The hallux is broad, and opposable to the other digits. The medius and index have a 

 common integumental covering, and the annularis is the longest of the four outer toes. 

 Dissection however, reveals certain important points of difference in the arrangement of 

 the intrinsic muscles. 



Plantar layer. — r The muscles composing this layer were found to differ in their 

 arrangement in the two specimens which were dissected. In one they closely resembled 

 the corresponding muscles in the Cuscus. Thus a median raphe extended from the base 

 of the second metatarsal, to the outer side of the base of the proximal phalanx of the index. 

 From this the adductor minimi digiti, the adductor annularis, and the adductor hallucis 

 took origin. The adductor of the ring digit, however, was very feebly developed. 



In the second specimen, the raphe stretched from the base of the middle metatarsal to 

 the inner side of the base of the proximal phalanx of the annularis. From this, three 

 muscles arose, either entirely, or in part, viz. : — 



1. The adductor minimi digiti. 2. The adductor indicis. 



3. The adductor hallucis. 



