150 ON THE MORPHOLOGICAL CONSTITUTION OF 



resembles these in being an ossified portion of the primordial 

 axis of the cranium, in being flattened into a horizontal plate 

 at its upper margin, in extending down to the line of the base 

 of the skull, and in thus presenting a catacentric relation to 

 its neural arch. The passage of the anterior acuminated ex- 

 tremity of the centrum behind beneath the lower margin of 

 the pre-sphenoidal centrum, so as to support it, is merely an 

 example of that longitudinal obliquity in the setting of cranial 

 centrums against one another, which may be considered as the 

 rule rather than an exception. The posterior margin of the 

 bone is oblique from below upwards and forwards ; gives 

 attachment to the orbito-sphenoids, or to their membranous 

 neurapophyseal substitutes, which bound or give passage to 

 the orbital and olfactory nerves. The obhquity of this margin 

 of the bone corresponds with the similar obliquity of the fore 

 part of the basis of the brain of the bird, a remarkable feature 

 in its configuration. The flattened upper edge of the bone may 

 be more or less exposed on the upper surface of the cranium ; 

 and when the intermaxillaries, ethmoido- and sphenoido- 

 frontals are removed, this flattened margin is found to be 

 similar to and continuous with the flattened upper margin of 

 the ethmoidal and vomerine cartilaginous septum. The 

 anterior margin may be nearly perpendicular, but is generally 

 oblique from below upwards and forwards, concave or concavo- 

 convex, sharp, and generally free, being connected to the 

 posterior margin of the ethmoidal cartilaginous septum by 

 membrane, thus permitting, more or less, movement of the 

 upper mandible — that is of the combined ethmoidal and 

 vomerine sclerotomes on the pre-sphenoidal. 



In the majority of birds a laminar process projects out- 

 wards and downwards from the lower and fore part of this 

 bone. This process, variously developed, forms, along with 

 the descending process of the lachrymal, the anterior wall of 

 the orbit, separating it from the nasal space, and permitting 



