THE MOBMTEID^, NOTOPTEKID^, AND HTODONTIDiE. 207 



Mormyridse, bounded by the squamosal, epiotic, and exoccipital, 

 although there is probably some degree of morphological relation- 

 ship existing between the two. It finds its nearer equivalent in 

 the lateral depression (pre-epiotic fossa), which in Osmemis, 

 Clupea, &c. is situated immediately in front of the vertical 

 posterior edge of the epiotic bone. 



At the side of the base of the cranium, and below the level 

 of the squamoso-opisthotic ridge, is a great vesicle of the swim- 

 bladder. Its outer wall is composed of fibrous tissue, which 

 is attached to the cranium along the line marked with dots 

 in fig. 20. Its inner wall is formed by the exoccipital and basi- 

 occipital, and its anterior wall by a vertical lamina of the pro-otic. 

 Between the exoccipital, basioccipital, aud pro-otic is a fairly 

 large auditory fenestra *, opening into the perilymphatic cavity, 

 and traversed vertically by the pro-otic lamiua just mentioned. 

 The foramen for the vagus, situated dorso-posteriorly to tlie 

 vesicle of the swim-bladder, is of large size. 



On separating the cranium from the vertebral column, the 

 basioccipital and the exoccipitals part readily from the half- 

 centrum, and are left exhibiting a rough surface with the usual 

 triradiate suture. The vertebral body t is not shorter antero- 

 posteriorly than the centra that follow, and it is provided with a 

 complete neural arch, the paired spinous processes of which are 

 but little smaller than those of the succeeding vertebra?. 

 Syodon must thus be regarded as one of the instances in which 

 the tendency for the appropriation of the first centrum by the 

 basicranial axis is of the slightest character. 



The basisphenoid is situated farther forward than usual, and 

 has no vertically descending process. It extends so far forward 

 beneath the alisphenoids as to touch the orbitosphenoid, and it 

 forms the superior edge of the optic foramen. The orbito- 

 sphenoid is U-shaped in section, and, at least in the dried skull, 

 the cranial cavity continues through it to the ethmoid region. 

 The orbitosphenoid nearly touches the prefrontals, but is 

 separated from them by a narrow tract of cartilage. Tlie 

 interorbital septum below the orbitosphenoid and basisphenoid 

 is membranous. 



* The auditory fenestra is a Clupeoid feature, and its occurrence in Hyodon 

 is of some interest. 



t It is not shown in figures 19, 20, and 21. 



