MORPHOLOGY OF OPISTHOCOMUS CfilSTATUS. 79 



the lesser fibulare have not yet appeared as bony centres ; they commence two or three 

 weeks after the intermedium, and are formed as central endostoses, like the bony matter 

 in the lower tarsal mass. This anachronism as to the ossifications of these three parts 

 is very remarkable, and the type of ossification is entirely different ; the intermedium 

 is a shaft-bone, whilst the tibiale and fibulare, as in tarsals and carpals generally, are 

 ossified like epiphyses. 



The toes at the 1st stage (PI. VII., dg.^"^) are already developed; the second is a 

 little shorter and also stouter than the fourth ; the third is very long, but its proximal 

 and distal phalanges are smaller than those of the first toe or hallux ; the size of this 

 heel is very remarkable. The ossification of the ungual phalanges is at the end and 

 not in the middle, as in the others. The so-called ascending process of the tibiale 

 requires an ontological explanation ; its ossification, I am quite satisfied, is an inter- 

 medium ; and this, I take it, is not a tarsal, but belongs to the leg, as the intermedium 

 in the wing does to the forearm. 



The intermedium, in the generalized fore-paddle of certain extinct Eeptiles, is seen to 

 reach up to the humerus, between the radius and ulna (see Marsh on " Sauranodon," 

 Amer. Journ. of Sci. vol. xix., Feb. 1880, p. 170, fig. 1 ; and D'Arcy W. Thompson, Journ. 

 of Anat. & Phys. vol. xx. pp. 1-4). Even in Birds, and those of the higher sort, I find, 

 in some cases, the wedge of the intermedio-radiale, which grows upwards between the 

 radius and ulna, elongated and segmented ofi', as in the nestling of Spizella imsilla, 

 and in another species of S])izella I found the apex, representing the intermedium, 

 separately ossified from the radiale. 



In Opisthocomus, as we have seen, and in some other birds, the intermedium of the 

 carpus is, for a short time, separate. We cannot derive the Bird from any known 

 EeptUian or Amphibian type ; the true ancestor of the Feathered Fowl did not 

 possess a cheiropterygium, but its limbs were ichthyopterygia ; and the same may 

 be said of the existing Amphibia, all known Eeptiles, and the Mammalia also i. 

 In the adult Opisthocomus the hind limb is stout and strong, like that of a Gallina- 

 ceous bird. The femur is well arched ; the tarso-tibia also is normal ; its cnemial 

 outgrowth is not large ; there is the usual tendon bridge below, and a shallow inter- 

 condyloid space for the short process of the tarso-metatarsus. The fibula is half the 

 length of the tibia; the centrale forms a notable pseudo-sesamoid. The tarso-metatarsus 

 has a strong grooved process behind its head, which was pre-formed in cartilage 

 developing posterior vertical ridges to serve as pulleys for the flexor tendons. There 

 is only one hole in this mass (Huxley, op. at. p. 309, fig. 12), as in Corythaix, and 

 also in Crax, and in the Gallinaceae generally. This hole is finished by a periosteal 



' The marvellous series of extinct Reptiles discovered in stratum under stratum has emboldened some 

 biologists in the matter of ancestry. Guesswork, however, is not science ; and we have no proof that certain 

 archaic forms begat others that are of a more recent date: we " have nothing to draw with, and the well is 

 deep." 



