286 



MARSUPIALIA. 



the calcaneum : a greater part of the super- 

 incumbent pressure is transmitted by a trans- 

 versely extended convex anterior surface to the 

 scaphoid and cuboid bones. This form of 

 the astragalus is also characteristic of the Koala, 

 Petaunsts, Dasyures, and the Pedimarious 

 Marsupials (d, Jig. 111). In the Kangaroos, 

 Potoroos, and Perameles which have the pedes 

 saltatorii, the fibular articular surface of the 

 astragalus is bent down as usual at nearly right 

 angles with the upper tibial surface. 



The calcaneum in the Wombat presents a 

 ridge on the outer surface which serves to sus- 

 tain the pressure of the external inalleolus which 

 is not articulated to the side of the astragalus. 

 The internal surface which joins the astraga- 

 lus is continuous with the anterior slightly 

 concave surface which articulates with the 

 cuboides. The posterior part of the bone is 

 compressed, it projects backwards for nearly 

 an inch, and is slightly bent downwards and 

 inwards. This part is relatively shorter in the 

 Koala, Phalangers, Opossums, and Petaurists, 

 but it is as strongly developed in the Dasyuri 

 as in the Wombat. The anterior part of the 

 calcaneum of the Phalangers is shown at e, 



/g. 111. 



In the Dasyurus macrurus I observe a small 

 sesamoidbone wedged in between the astragal us, 

 tibia, and fibula at the back part of the ankle- 

 joint. In the Petaurus taguanoides there is a 

 supplemental tarsal bone wedged in between 

 the naviculare and cuboides on the plantar sur- 

 face. In the hand-like foot of the Phalanger 

 the structure of the tarsus is shown in jig. Ill : 

 ./"is the naviculare, g the internal cuneiform, and 

 h the os cuboides. In the Wombat the sca- 

 phoid, cuboid, and three cuneiform bones have 

 the ordinary uses and relative positions. 



The analogy of the carpal and tarsal bones 

 is very clearly illustrated in this animal. The 

 anchylosed naviculure and lunare of the hand 

 correspond with the astragalus and navicu- 

 lare of the foot, transferring the pressure of 

 the facile, majus upon the three innermost 

 bones of the second series. The long, back- 

 ward-projecting pisiform bone of the wrist 

 closely resembles the posterior process of the 

 os calcis; the articular portion or body of 

 the os calcis corresponds with the cuneiforme 

 of the carpus ; the large carpal unciform re- 

 presents the tarsal cuboides, and performs the 

 same function, supporting the two outer digits ; 

 the three cuneiform bones of the foot are obvi- 

 ously analogous to the trapezium, trapezoides, 

 and os magnum. The internal cuneiform bone 

 is the largest of the three in the Wombat, al- 

 though it supports the smallest of the toes. It 

 is of course more developed in the Pedimanous 

 Marsupials, where it supports a large and op- 

 posable thumb. 



In the Wombat the metatarsals progressively 

 increase in length and breadth from the inner- 

 most to the fourth ; the fifth or outermost meta- 

 tarsal is somewhat shorter but twice as thick, 

 and it sends off a strong obtuse process from 

 the outside of its proximal end. A correspond- 

 ing process exists in the Phalangers (jig. 111). 

 The innermost metatarsal of the Wombat (Jig. 



105) supports only a single phalanx ; the rest 

 are succeeded by three phalanges each, pro- 

 gressively increasing in thickness to the outer- 

 most ; the ungueal phalanges are elongated, 

 gently curved downwards, and gradually dimi- 

 nish to a point. 



In the Myrmecobius the tibial or inner- 

 most toe is represented by a short rudimental 

 metatarsal bone concealed under the skin. 

 In the Dasyures the innermost toe has two 

 phalanges, but it is the most slender and does 

 not exceed in length the metatarsal bone of the 

 second toe. In the Petaurists it is rather 

 shorter than the other digits but is the strongest, 

 and in Petaurus taguanoides the terminal 

 phalanx is flattened and expanded ; the toes are 

 set wide apart in this genus. In the Opossums 

 and Phalangers the innermost metatarsal bone 

 is directed inwards apart from the rest, and 

 together with the first phalanx is broad and flat. 

 The second phalanx in the Opossums supports 

 a claw, but in the Phalangers is short, trans- 

 verse, unarmed, singularly expanded in Ph. 

 Cookii, but almost obsolete in Ph. ursina 

 (jig- 111,1). In all the preceding genera there 

 are two small sesamoid bones on the under 

 side of the joints of the toes, both in the fore 

 and hind feet. 



The commencement of a degeneration of the 

 foot which is peculiar to and highly character- 

 istic of the Marsupial animals may be discerned 

 in the Petaurists, in the slender condition of 

 the second and third toes, as compared with 

 the fourth and fifth. In the Phalangers this 

 diminution of size of the second and third toes, 

 counting from the hallux, is more marked. 

 They are, also, both of the same length and 

 have no individual motion, being united to- 

 gether in the same sheath of integument as far 

 as the ungueal phalanges, whence the name of 

 Phalangiata applied to this genus (fg. I'll, 

 2 and 3). 



In the saltatorial genera of Marsupials the 

 degradation of the corresponding toes is ex- 

 treme, but though reduced to almost fila- 

 mentary slenderness they retain the usual num- 

 ber of phalanges, and the terminal one of each 

 is armed with a claw. These claws being the 

 only part of the rudimental digits which pro- 

 ject freely beyond the integument, they look 

 like little appendages at the inner side of the 

 foot for the purpose of scratching the skin and 

 dressing the fur, to which offices they are 

 exclusively designed. The removal of the in- 

 nermost toe, corresponding with our great 

 toe and the hallux of the Pedimana, commences 

 in the Perameles. In one species I find the 

 metatarsal bone of this toe supports only a 

 single rudimental phalanx which reaches to the 

 end of the next metatarsal bone, and the inter- 

 nal cuneiform bone is elongated. In another 

 species the internal toe is as long as the abortive 

 second and third toes, and has two phalanges, 

 the last of which is divided by the longitudinal 

 fissure characteristic of the ungueal phalanges 

 in this genus. In the Perameles lagotis the 

 innermost toe is represented by a rudimentary 

 metatarsal bone, about one-third the length of 

 the adjoining metatarsal. 



