NARRATIVE nF THE CRUISE. 157 



plates also arc devoted to the osteology of Thylacinua vynocephalus. Owing to the 

 prevalent belief that the genus, of which this animal is the sole member, is rapidly 

 becoming extinct, special care has been taken in recording its anatomical peculiarities. 



The chief interest of the Report, however, is centred in the fact that it contains 

 the results of an investigation into the comparative anatomy and homologies of the 

 intrinsic muscles and nerves of the Mammalian foot. The author was induced to engage 

 in this research from conditions which he found in the Marsupial foot, and special oppor- 

 tunities were afforded him for carrying on the work by the many valuable specimens of 

 Mammalia collected during the voyage, which were also placed at his disposal. 



The conclusions arrived at may be briefly stated to be the following: — That the typical 

 anrangement of the intrinsic muscles of the Mammalian pes is seen to best advantage 

 in the feet of certain of the Marsupialia. In these animals the muscles are disposed in 



Fio. 171. — Schematic view of a section through the metatarsus of a typical mammalian foot. 

 I.-V. Metatarsals; y 1 -^ 5 , adductores ; /*-/*, flexors* breves; cP <J* abductores; e.p.n., external plantar nerve. 



three layers, viz., (1) a plantar layer of adductores, (2) an intermediate layer of flexorcs 

 breves, and (3) a dorsal layer of abductores. 



Deviations from this typical trilaminar disposition may take place — (a) by subdivision 

 of certain of the members of one or other of the layers, (b) by fusion of certain of the 

 elements of the different strata, or (c) by suppression or non-development of some of the 

 muscles. 



The first of these deviations is to be found in a few Marsupial animals (e.g., Cuscus) 

 in which a tendency is exhibited to the development of a fourth layer by the splitting of 

 the dorsal interossei. Fusion of the constituents of the intermediate and dorsal layers is 

 extremely common, whilst fusion between the plantar and intermediate muscles is a very- 

 rare occurrence. Suppression of certain of the muscles may take place in two ways ; it 

 may either be complete — not a trace of the lost muscle being left — or partial, in which 

 case the place of the missing muscular belly is taken by a ligamentous structure, having 



(narr. chall. exp. — vol. i. — 1884.) 58 



