THE IGUANODON AND H YL^EOSAURUS. 
281 
From the outer tubercle of the head to the external condyle . . 33 inches. 
Circumference of the head 23^ 
round the condyles 21^ 
Circumference of the shaft at the deltoid crest 
one-third from the distal extremity 16 
The medullary cavity only extends to within one-third of the top of the bone ; it is 
3 inches in diameter: the greatest thickness of the wall of the shaft is 1 inch. 
The head of the bone presents the usual posterior protuberance of the humerus in 
Lizards, but the epiphysis of this, as well as of the distal extremity, is wanting, as is the 
case in all the long bones of the Wealden reptiles. At about 3 inches from the top the 
ridge or crest for the insertion of the deltoid muscle (d) is considerably developed, and 
extends 15 inches down the shaft, which rapidly contracts below, and finally expands 
to form the condyloid extremity. The articular face of the latter (Plate XXXI. fig. 1 9"^) 
is divided into two nearly equal condyles ; the inner or ulnar segment (e) is traversed 
by an anterior furrow, which is more strongly marked in the humerus of a younger 
individual (Plate XXXI. fig. 18“^ e) : the posterior or olecranal fossa (g) is simple, and 
somewhat deeper than the anterior. On the whole, the aspect of this humerus more 
closely corresponds with that of the Crocodiles than of the ordinary Lizards. 
I have for many years possessed the head or proximal extremity and the lower or 
condyloid end of two humeri, which must have belonged to very young Iguanodons. 
The former is of a left humerus; it is 8 inches in circumference, and with the excep- 
tion of the absence of the epiphysis, is remarkably perfect ; it is identical with the 
large specimen, and is figured one-third its natural size, Plate XXXI. fig. 21. The 
specimen of the lower or distal end of a right humerus is represented, Plate XXXI. 
fig. 18; it beautifully displays the condyloid facet for articulation with the bones of 
the fore-arm. In all these fossils the medullary cavity is large, and extends to within 
about one-third of the top. 
In the same plate (Plate XXXI. fig. 20) is given a figure of the humerus in the Maid- 
stone specimen, one-sixth its natural size; the situation of this bone is pointed out 
in the outline* of the scapula and adjacent bones. Thus after the lapse of fifteen 
years two important elements of the skeleton of the Iguanodon contained in that 
most valuable fossil, are now for the first time determined. The small size of the 
humeri, as compared with that of the femora, seems at first to present an objection 
to this interpretation ; but the difference is not greater than obtains in many other 
fossil Saurians'l', as well as in recent Lizards. The length of the Maidstone humerus 
* Ante, p. 279, fig. 1. 
t “ C’est un fait a peu pres general que les membres ant^rieurs des reptiles crocodiliens et lacertiens sont 
plus courts et plus faibles que les posterieurs ; chez quelques especes la difference est tres-prononcee. Mais 
nos reptiles fossiles des environs de Caen annoncent une disproportion beaucoup plus forte encore entre ces 
membres : le Pcekilopleuron, le Steneosaurus de Quilly, les Teleosaurus, en fournissent la preuve. Ces der- 
niers surtout avaient les membres anterieurs d’une excessive petitesse ; les deux paires de membres dilferaient 
entre elles plus peut-etre qu’elles ne different les Gerbilles et les Kangaroos.” — Deslongchamps, Memoire sur 
le Pakilopleuron Bucklandii, p. 81. 
