434 



NOTES. 



I. On the Mastoid in Birds. 



The true mastoid of the bird seems hitherto to have escaped notice. 



Hallmann says (I. c. p. 33), "In the disarticulated skulls of chickens, I 

 examined the share taken by the different bones in the formation of the 

 labyrinth, by introducing bristles into the semicircular canals, and I 

 found in the proper petrosum (into which the facial and acoustic nerves 

 enter, and which contains the cochlea) the anterior cms of the anterior 

 canal (I term the upper one thus for ready comparison with reptiles) 

 and of the external canal; in the supraoccipital, the upper (= posterior) 

 crus of the anterior canal, and the upper end of the posterior canal j and 

 in the exoccipital, the lower crus of the posterior, and the posterior of the 

 external canal. In other words, the distribution of the canals is as in 

 the scaly Amphibia. For the rest, in birds as in mammals, and probably 

 in all Vertebrata, the membranous semicircolar canals are formed con- 

 nectedly in the cartilage, and the bony parts only gradually invest them. 

 Hence, when the chick's skull is too young, but very little of the posterior 

 canal is to be found in the supraoccipital, which in fact contains some- 

 what less of the posterior canal than of the anterior, and thereby departs 

 from reptiles and approximates mammals. 



"At a certain period also, an interval filled with cartilage, through which 

 the semicircular canals shine, is found in the bird's skull between the 

 supraoccipital, the parietal, the squama temporis, and the exoccipital. I 

 see this clearly in the skull of a young Dicholophus cristatus (No. 5605, 

 B. M.). In the skeleton of a young Colymbus cristatus (No. 7172, B. M.), 

 I find that this interval is, on the right side, almost filled up by a small 

 bony plate, which has not as yet combined with the surrounding bones. 

 This appears to me to be a separate pars mastoidea, which however com- 

 bines very early with the exoccipital. In the skull of a young goose 

 (fig. 3) (No. 3507, B. M.), this distinct piece (e between s, r, t and I) 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 Hall- 

 mann 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. 



II. On the influence 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, 



