435 



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

 p. 171). 



Professor Owen (Principes d'Oste'ologie Comparee, p. 55) adopts Cu- 

 vier's argument, and pushes it further, endeavouring to show that the dis- 

 appearance 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 neces- 

 sary consequence of his identification of the true petrosal with the ali- 

 sphenoid. 



It seems strange that Cuvier should have advanced so weak an argu- 

 ment as that which I have cited j for assuredly Euminants 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 dis- 

 appears from the interior of the skull. Kb'stlin 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 Cheiro- 

 ptera, Insectivora, and a few Marsupialia, 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 Pro- 

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

 squamosal from the interior of the skull causes the alisphenoid to pass 

 behind the exit of the trigeminal, this retrogression ought to have taken 

 place in the ostrich. Nothing of the kind has occurred, however, the 

 trigeminal foramen being a 'trou de conjugaison' 1 between the alisphenoid 

 and the petrosal. It does not even traverse the middle of the ali- 

 sphenoid, as in the sheep. 



It is unnecessary to discuss the effect of the disappearance of the pe- 

 trosal, as I have endeavoured to prove that it does not disappear in the 

 lower Vertebrata. 



III. Connexions of the tympanic membrane in jBirds. 

 According to Kostlin (/. c. p. 216), the tympanic membrane of birds is 



* Alisphenoid and petrosal, rnihi. 



