S86 ON THE THEORY OF THE VERTEBRATE SKULL 



2. The basis cranii is never segmented. 



3. The lamina perpendicularis of the ethmoid has the same mor- 

 phological value as the presphenoid. 



4. The petrosal has the same morphological value as the mastoid ;. 

 if one is not an integral part of the skull, neither is the other. 



5. The nasal bones are not neurapophyses. 



6. The branchial arches have the same morphological value as the- 

 hyoid, and the latter as the mandibular arc. 



7. The mandibular arc is primitively attached behind the point of 

 exit from the skull, of the third division of the fifth nerve. 



8. The premaxilla is originally totally distinct from the palato- 

 maxillary arcade. 



9. The pectoral arch is originally totally distinct from the skull. 

 Starting on this basis, it might not be difficult to show that the 



perfectly ossified skull is divisible into a series of segments, whose 

 analogy with vertebrae is closer the nearer they lie to the occipital 

 region ; but the relation is an analogy and not an affinity, and these- 

 cephalic sclerotomes are not vertebrae. 



NOTES. 



L — On the Mastoid in Birds. 



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



Hallmann says (/. 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 intro- 

 ducing bristles into the semicircular canals, and I found in the proper fetrosuin 

 (into which the facial and acoustic nerves enter, and which contains the cochlea) 

 the anterior crus of the anterior canal (1 term the upper one thus for ready com- 

 parison 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 ; 

 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 

 Ainpliibia. For the rest, in birds as in mammals, and probably in all Vertebrata, 

 the membranous semicircular canals are formed connectedly 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 somewhat 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 Dicholophiis 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 



