142 ON THE MOKPIIOLOGICAL CONSTITUTION OF 



of the pre-sphenoidal sclerotome converted into a mesial plate 

 extending up to and flattened out at the upper surface of the 

 cranium, in accordance with the catacentric character of the 

 neural arch of the sclerotome, of which it is an element. Its 

 corresponding neurapophyses are the pre-sphenoidal wings — 

 the " orbito-sphenoids" of Professor Owen — which not only- 

 bound laterally the orifices for the optic, but also those for the 

 olfactory nerves. The so-called " ethmoid" of the bird is not 

 therefore formed by the coalescence of a pair of " pre-frontals," 

 but is a mesial element belonging to another sclerotome. The 

 bird already possesses distinct or "divided" "pre-frontals," 

 with all the characters of the " pre-frontals" of the reptile in 

 its so-called " nasals." 



Du£j^s considered the " os en ceinture " of the froo- to be 

 the ethmoid, from its giving passage to the olfactory nerves 

 by two funnel-shaped orifices at its anterior extremity, and 

 from its intimate connection with the nasal cartilage in front. 

 Professor Owen, on the same grounds, while he holds the pos- 

 terior part of this bone in the Bana toans to consist of the 

 " orbito-sphenoids," looks upon its anterior part as the confluent 

 " pre-frontals." But as the " os en ceinture " of the common 

 fros originates in a centre of ossification on each side of its 

 fundamental portion of the primordial cranium. ; and as Pro- 

 fessor Owen does not state the grounds on which he holds the 

 " orbito-sphenoids " to be confluent with it in the bull-frog ; 

 as I can find no trace of such confluence either in the bull-frog 

 or common frog, and as the fore part of the bone is divided by a 

 mesial septum, — I look upon it as consisting of a single pair 

 of neurapophyses and a catacentric septum. As this " os en 

 ceinture " is situated upon the upper surface of the anterior 

 acuminated portion of the centrum of the post- sphenoid, as in 

 the bird, and as it is covered above, and in the common frog 

 is united with the anterior portion of the so-called " parieto- 

 frontal," it appears to me to constitute tlie neural arch and 



