ON THE THEORY OF THE VERTEBRATE SKULL 587 



with the surrounding bones. This appears to me to be a separate /arj mastoidea,. 

 which however combines very early with the exoccipital. In the skull of a 

 young goose (fig. 3) No. 3507, B.M.), this distinct piece {e between j-, r, t and /) 

 is still better shown. Subsequently it is altogether indistinguishable from the 

 exoccipital." 



I have endeavoured to show, however, that the true mastoid of the bird is to be 

 sought elsewhere ; and at any rate the bone described by Hallmann has not those 

 relations which he himself considers essential for a mastoid. It appears to me, 

 that the distinct ossification he mentions is the epiotic bone, which has not yet 

 combined with the surrounding parts. 



\\.—0n the influetice of the share taken by the squamosal in the Vertebrate Skull. 



In discussing the homologies of the bones of the skull of the crocodile, Cuvier 

 ('Ossemens Fossiles,' t. ix. p. 163) states that "the squamosal and zygomatic bone 

 becomes more and more excluded from the cranium as we descend in the scale of 

 quadrupeds, so that in Ruminants it is rather stuck upon the skull than enters into 

 the composition of its walls ; " and it is by this argument mainly that the great 

 anatomist justifies his identification of the quadratojugal of the crocodile with the 

 squamosal, or rather with the zygomatic portion of that bone in mammals (/. c. 

 p. 171). 



Professor Owen ('-Principes d'Osteologie Comparee,' p. 55) adopts Cuvier's 

 argument, and pushes it further, endeavouring to show that the disappearance 

 of the squamosal, and as he supposes of the petrosal, from the interior of the skull 

 in Reptilia, is sufficient to account for that retrogression of the alisphenoid behind 

 the exit of the fifth nerve, which is the necessary consequence of his identification 

 of the true petrosal with the alisphenoid. 



It seems strange that Cuvier should have advanced so weak an argument 

 as that which I have cited ; for assuredly Ruminants are not very low in 

 the mammalian scale, nor are they those mammals which most nearly approach 

 reptiles or birds. We must seek these among rodents and monotremes, in both of 

 which the squamosal enters largely into the composition of the cranial walls. 



This is particularly the case in that especially reptilian mammal the Echidna. 

 As to birds, it can still less be said that their squamosal disappears from the 

 interior of the skull. Kostlin says on this point (' Bau des knochernen Kopfes, 

 p. 206), " The squamosal contributes a small surface to the ridge, which separates 

 the anterior cranial fossa from the middle one. It is here applied above against 

 the parietal, anteriorly against the anterior, and posteriorly against the posterior 

 sphenoidal ala,^ and seems in all birds to appear at this point in the cavity of the 

 skull. In the goose its extent is far smaller than in the fowl. The actual size of 

 the ala temporis, however, surpasses that of its inner surface by a great deal. In 

 this respect birds are analogous to the Cheiroptera, Insectivora, and a few 

 Marsiipialia, where only a small portion of the squama temporis projects into 

 the cranial cavity. Still more do they resemble the seals, in which this part is 

 entirely enclosed by the parietal and ala temporis, and so is completely separated 

 from the petrosal." Kostlin is in error, however, in assuming that the squamosal 

 is visible in the interior of the skull of all birds, for as we have seen above, such is 

 not the case in the ostrich. 



The struthious skull then affords an important test of the value of Professor 

 Owen's argument. If, as he supposes, the disappearance of the squamosal from 



' Alisphenoid and petrosal, mihi. 



