165 



trum to the segment in front of the vomer. He considers the skull 

 to be composed of seven segments ; that three of these are developed 

 for protection of the encephalon, viz. posteriorly the occipital seg- 

 ment, then the parietal segment, to which belong the postsphenoid 

 and parietals, and anteriorly the frontal segment, to which the 

 frontal and central plate of the ethmoid belong; and that three 

 other segments with imperfect neural arches, and each connected 

 with a special sense, alternate with the first three, viz. the auditory, 

 optic, and olfactory segments, the last-mentioned being the vomerine 

 segment ; and that foremost of all is a terminal seventh segment, the 

 facial segment, to which the intermaxillaries, maxillaries, and nasals 

 belong. 



With a special view towards determining the exact construction of 

 the facial segment, he examines the anterior portions of the skulls 

 of birds, reptiles, and fishes. Taking as his guide the constitution 

 of the vomerine segment as he believes he has established it in 

 mammals, he concludes that the vomer in birds, reptiles, and fishes 

 is the bone described as such by Professor Owen ; that the lateral 

 masses of the ethmoid are the prefrontals of reptiles and fishes, and are 

 absent from the skull of birds ; that the central plate of the ethmoid 

 is represented by the interorbital septum (whether ossified or not) 

 of birds, reptiles, and fishes ; and that in fishes the nasals are repre- 

 sented by the nasal of Owen, which, however, usually contains an 

 additional element, which plays the part of centrum, and corresponds 

 to the mesial palatine processes of the intermaxillaries of other classes. 

 His examination of the skulls of birds, reptiles, and fishes has con- 

 vinced him that throughout the vertebrata the lateral plates of the 

 intermaxillaries form the proximal part of a haemal arch, of which 

 the maxillaries compose the distal part, while the nasals are elements 

 of the neural arch of the same segment ; that thus in mammals the 

 ring of bone surrounding the incisive foramina is the first haemal 

 arch ; that the nostrils in all vertebrata are openings situated be- 

 hind the facial and in front of the vomerine segment ; and that the 

 alar cartilages of the nose are structures serially homologous with the 

 tarsal cartilages and the pinnse of the ears. 



In attempting to explain the exact morphological relations of the 

 nasals, the author enters into the embryological bearings of the con- 

 clusions at which he has arrived, and puts forward the hypothesis 



