OF GALLINACEOUS BIRDS AND TINAMOUS. 221 



head, but oblique ; and its corresponding facet on the skull is extended along the side 

 of the prootic, over the opisthotic and the fenestra ovalis, to be finished by the lateral 

 occipital. 



In all birds, save the Gallinse, the head of the os quadratum leans backwards con- 

 siderably, and the lateral occipital generally contributes something towards the articular 

 concavity. In the typical birds the posterior part of the head is also the inner part, the 

 " OS quadratum " standing nearly at a right angle to the axis of the skull : to some 

 extent this is the case in the Ostriches also. In the transverse position of the os qua- 

 dratum the typical bird comes nearest the Lizard, but not in another and more essen- 

 tial point. The upper condyle of the os quadratum of the Tinamou, like that of the 

 Ostriches, although oblong and greatly turned backwards by the curving of the neck 

 of the bone, yet is quite simple ; there is no division into an external or anterior head 

 and an internal or posterior. Once above the Ostriches andTinamous, and we have the 

 articular condyle divided into two facets, even where the head of the bone is so little 

 changed from the hemispherical shape as in the Fowls and Parrots. The more this 

 head is divided, the nearer do we come to the " incus " of the mammal with its anterior 

 or outer crus, and its posterior or inner — the former in the embryo having its own 

 " acetabulum " on the side of the cartilaginous prootic region, the latter maintaining 

 its separateness from the skull-wall, and soon contracting an articulation with the 

 " stapes." 



But there is a little bone which intervenes between the incus and stapes in the 

 mammal ; has it any separate existence in the bird ? I believe not ; but Professor 

 Huxley, in his late ' Hunterian Lectures ' (see ' Lancet,' April 18th, 1863, p. 435), has 

 suggested that it may possibly have its homologue in that bone of the fish which Cuvier 

 called " tympanal," but which he calls " metapterygoid." This fish-bone is merely a 

 large superquadrate epiphysis, and is never separated by an articulation from the os 

 quadratum ; only it starts from its own osseous centre, and a large clear tract of unaltered 

 cartilage always separates it from the os quadratum. This is nothing more than the ossi- 

 fied end of the palato-quadrate arch of the embryo; and in Fishes the posterior crus of 

 this arch runs upwards to the skull, and is of considerable length and breadth. Now, in 

 the Lacertilia generally, and in the BUndworms (Anguis), the posterior crus of this arch 

 is also of great relative length, and only its thick lower part and the crown of the arch 

 is used up in the formation of the os quadratum : the upper pointed part of the crus 

 becomes detached, is ossified slowly and imperfectly, and gets firmly wedged between 

 the lower end of the squamosal, the tip of the long parietal horn, and the end of the 

 lateral occipital process. In some Lizards, e. g. Trachydosaurus rugosus, a flat, cake- 

 shaped epiphysis is formed on the end of this lateral occipital pedicle, and it intervenes 

 between the superquadrate epiphysis and the occipital spur. In the Chamaeleon the 

 parietal does not come down to this point. I have not seen this bone — the super- 

 quadrate or metapterygoid — in the Ophidians, Chelonians, Crocodilians, nor in Birds. 



