612 BRITISH FOSSIL REPTILES. 



extinct ones exemplify this relation. In the metatarsus of the Bird the shafts of the ento-,. 

 raeso-, and ecto-metatarsi are severally ossified from separate centres, but the proximal 

 epiphyses of the three bones are ossified from one centre, and form a single cap of bone 

 where the shafts are still distinct.* Such cap (Eig. 10, Dlnornis, p m) may be arbitrarily 

 homologised with one or more bones of the distal tarsal series in Reptiles (Eig. IG, ScelidO' 

 satcrus, b, e ; in Varanm, h, e) and in Mammals (Eig. 16, Mitminant, b, n, e). It seems more 

 natural to regard it as answering to the epiphysial cap, covering the ends of the two chief 

 metatarsals, of the Ruminant (ib. ib., 2m, Hi, iv), and I associate such instances of complex 

 osteogeny of the metatarsus with the high conditions of organisation differentiating the 

 warm-blooded classes, Aves and Mammalia, from the cold-blooded licpUUa. 



In the Ruminant, as in the Bird, the single epiphysis and multiple diaphyses coalesce 

 into one so-called ' cannon bone.' 



In the Dinosaiu'ia the hind limbs are not adapted, as in the Birds, for transference 

 of the entire Aveight of trunk, neck, head, and fore limbs, from the leg upon the foot by due 

 development and modifications of the main leg-bone, the tibia ; but the fibula is continued 

 to the ankle-joint, and takes a larger share in its formation than is usual in Mannnals. 

 Both leg-bones have their distal epiphyses (Fig. 16,^/,;, t. Scelidosaurus, Varanus). The 

 tarsal segment is represented, usually by four ossicles : one, a, answers, by its connections, 

 to the astragalus, naviculare, and entocuneiform bones of the Mammal ; a second, I, repre- 

 sents the calcaneum with the lever process slightly if at all developed ; there are, also, a 

 cuboid, I, and an ectocuneiform, e- The metatarsals, whether they be three or four 

 in number, never coalesce, but retain their primitive distinctness throughout life. The 

 sole ground taken to bridge over this significant difference in the structure of leg and foot 

 in the Bird and Dinosaur is to affirm that the distal epiphysis, pf, of the tibia in the Bird, 

 is the homologue of the astragalus in the Mammal and Reptile (Fig. 16, a).t 



" If the whole hind-quarters, from the ilium to the toes, of a half-hatched Chicken 

 could be suddenly enlarged, ossified, and fossilised as they are," | the ilium would be 

 distinguished from that of a Dinosaur by the major number of its sacrovertebral attach- 

 ments and by their greater extent, by the absence of the ridge continued from the super- 

 acetabular plate upon the antacetabular one ; the pelvis would be distinguished by the 

 presence in the ischium of an obturator process wanting in the Dinosaur (Eig. 12, is), and 

 by the absence of a pectineal process of the pubis present in the Dinosaur (ib., pi), by 

 the parallelism of the ischium and pubis, and by the backward extension of both bones 

 (compare Eigs. 12 and 14). The differences grow and multiply as the comparison proceeds ; 

 as, e.(/., by the non-extension, in the Chick, of the fibula (Eig. 14,/i) to the ankle-joint and 

 by the larger and more complex distal epiphysis of its tibia (Eig. 16, Binornis), by the 



* 'Transactions of the Zoological Society of London,' 4to, vol. iv (1856), p. 149, pi. xlv {Dinornis 

 elephantop^is, puUus ; Binornis crassvs, pullus). 



t Prof. Huxley, ' Quarterly Journal Geol. Soc.,' vol. .xxvi, p. 29. 

 ; X lb., loc. cit., p. 30. 



