1910.] Manu.^ of Echidna, Didrlpltis: (he Ancestral Placeyitah. 443 



contact with the intcniicduiui. Digit V in Oiulriiodoii is nearly in contact 

 with the ulnare (cuneiform); in the embryo Echidna it actually touches it. 



(2) The whole manus is spreading and fan-like, suggesting that of 

 Oudenodon and digits I and V likewise diverge sharply. The divergence 

 of digit I and the spreading character of the hand in Oudenodon and the 

 embryo Echidna as well as in the embryos of many higher mammals is 

 esjH'cially interesting because it shows that the hand of the ancestral mam- 

 mals hatl the potentiality of adaptation into either (a) the digging and 

 swimming type or (6) the climbing and grasping type or (c) the ambulatory 

 and finally cursorial types. 



(3) In other characters the manus of the embryo Echidna is more like 

 that of Theriodesmus (P'ig. 28, no. 5). The scapho-lunar-centrale complex- 

 might be derived from the arrangement of the radiale, intermedium and cen- 

 trale 1 in that genus; centrale 2 might have coalesced with the unciform (4). 

 The intermedium (lunar) in Echidna articulates chiefly with the radius, a 

 mammalian character, wheras in Therapsids it occupies the space between 

 the radius and ulna. 



(4) A radial sesamoid (Fig. 28, no. S) which occurs also in the embryo 

 Didelphis and in many higher mammals, is present. 



Didelphis. — The manus of the embryonic Didelphis figured by Emery 

 (Fig. 28, no. 8) is of a somewhat more normal mammalian type, but none 

 the less is clearly derivable from the type seen in Theriodesmus, which it re- 

 sembles in the relations of the pmehallux (Fig. 28, no. 5) and of the distal 

 carpalia 1-4. The intermedium (lunar) is now entirely overspread l)y the 

 radius and has possibly united with centrale 2. Centrale 1 may have united 

 with the scaphoid or with the lunar but at any rate it is never free in Marsu- 

 pials. In the Marsu})ials also the magnum and unciform are generally larger 

 than the lunar and cuneiform, whereas in most primitive Placentals the re- 

 verse is the case. 



The lunar in Didelphis is in broad contact with the uncifonn, as is the 

 case in the great majority of the primitive mammalian types. As compared 

 with the embryonic manus of Echidna, that of Didelphis shows how readily 

 the grasping-climbing type may be derived from the primitive fan-shaped 

 paddle-like type. 



Ancestral Placentals-. — Xo mammalian manus is known before the 

 Basal Eocene; consequently the morphological gap between the manus 

 of the Therapsida and the manus of early Placentals has not yet been bridged 

 over by discovery. We may feel certain, however, that the pentadactyl 

 manus of the ancestral Placental must have had a more or less divergent 

 pollex, a large trapezium, a free centrale, and a relatively small magnum, 

 because these characters are preserved not only in the Basal Eocene Creo- 



