1884.] MR. J. B. SUTTON ON THE CRANIAL ARCADE. 567 



basi-sphenoid, and ethmoidal cartilage ; it extends the whole length 

 of the ethtno- vomerine cartilage, and at its anterior end i-ests upon a 

 median-placed bone carrying teeth, and usually marked "vomer." 



The vomer of a human foetus at the time of birth is represented 

 in Plate LIII. fig. 3. 



It arises from a single ossific nucleus which is deposited in the 

 perichondrium covering the lower border of the ethmo-vomerine plate. 

 It underlies the basi-sphenoidal and ethmoidal regions, and lodges 

 the ethmo-vomerine cartilage in a shallow trough ; anteriorly it ends 

 by resting on two horizontal plates of bone, which in the usual 

 condition of the parts constitute elements of the complex superior 

 maxillary bone. 



The vomer is represented in situ, Plate LIII. fig. 4, and the para- 

 sphenoid in fig. 1, where it wiil be observed that, so far as shape, 

 situation, and relations go, the two bones correspond in a remarkable 

 manner. 



If, instead of taking a human foetus at birth, we examine one at 

 an earlier stage, before the hard palate is completed, we shall find 

 that the mouth and nasal fossae form one common cavity, the vomer 

 forms a partial covering to the ethmo-vomerine plate, and rests 

 anteriorly on the premaxillae. In this condition it still more 

 strikingly compares with the Pike's parasphenoid. Indeed, its 

 homology is self-evident. On the other hand, to make the Pike's 

 parasphenoid compare with the vomer of a mammal at birth, it is 

 only necessary to widen the bones forming the anterior part of the 

 palato-quadrate arch so as to meet the parasphenoid, or, coalescing 

 in the median line, shut it off from the buccal cavity. 



To my mind the most important feature that surrounds the 

 parasphenoid (vomer) seems to be this : — 



In crania possessing no bony basi-occipital or ossified sphenoidal 

 region in the base of the skull, this bone plays the part of a keel to 

 the brain-case and face (in Menobranchus it constitutes the floor of 

 the cranium), and forms an important element in the structure of the 

 skull. As ossific nuclei begin to be deposited in the basal cartilage, 

 such bones as the basi-occipital and the basi- and pre-sphenoid 

 are support enough in themselves ; thus the function of the para- 

 sphenoid (vomer) becomes abrogated and commences to lose its 

 relatively great importance. Take for examples Menobranchus, 

 Siredon, &n& Siren, all of which lack a basi-occipital bone ; in each of 

 these instances the parasphenoid (vomer) extends as far backwards 

 as the anterior boundary of the foramen magnum. In the Pike, 

 Cod, and such fish possessing an ossified basi-occipital, the vomer 

 only ventures as far backwarda as the anterior end of that bone, 

 merely for greater support. Ascending the scale of vertebrates and 

 gaining mammals, we find well-ossified bones in the skull's base, of 

 cartilage origin, strong and capable supports for the skull-vault ; and 

 the vomer is reduced to slender and delicate proportions, merely 

 performing the insignificant duty of forming a part of the septum 

 between the neighbouring nasal passages. 



In the skull of the Frog, however, this median parasphenoid 



