420 Royal Society : — 



in the mammal. This is least the case with the largest and upper- 

 most of these bones, which lies upon the parietal above, the aU- 

 sphenoid in front, and the exoccipital behind ; while internally it is 

 in relation with the petromastoid. 



This bone lies immediately above an articular surface, which is 

 furnished to the os quadratum by the petrosal, and more remotely 

 it helps to roof-in the tympanic cavity, but takes no share in the 

 formation of the fenestra ovalis. It sends a free pointed process 

 downwards and forwards, which does not articulate with the jugal. 

 Except in this particular, however, the bone in question resembles 

 in every essential relation the squamosal of the sheep, while to the 

 same extent it differs from the mastoid of that animal. 



I have stated that in the ostrich this bone does not appear upon 

 the inner surface of the wall of the skull, and in this respect, while 

 it resembles the squamosal of the sheep and Ruminants generally, 

 it differs from that of most other Mammalia, in which the squa- 

 mosal makes its appearance in the interior of the skull, between 

 the parietal, frontal, alisphenoid and petrosal bones, and so con- 

 tributes more or less largely to the completion of the cranial wall. 



But it has been most strangely forgotten that the relations of 

 the bone in question in birds are by no means always those which 

 obtain in the ostrich. In the young of the commonest and most 

 accessible of domestic birds, in the chicken, the squamosal may be 

 readily seen to enter largely into the cranial wall, — a rhomboidal 

 portion of its anterior and internal surface being interposed in front 

 of the petrosal, between this bone, the parietal, the frontal, and the 

 alisphenoid (Sq. fig. 2). 



B.a' ^ "■■ 



Fig. 2. — Longitudinal section of the Skull of a young Chicken. 



There is therefore not a single relation (save the connexion of 

 the jugal) in which this bone does not resemble the squamosal of the 

 Mammalia — there is not one in which it does not differ from their 

 mastoid. 



The second bone applied externally to the cranium in the bird, 

 is that large and important structure, the os quadratum, which in- 

 tervenes between the petrosal and squamosal bones above, and the 

 articular portion of the lower jaw below, — which articulates with the 



