114 



MR. E. PHELPS ALUS, JUN., ON 



prarcerebralis, and hence not parts of the Prsefrontalliieke of 

 Gegenbaur's (1872) descriptions of the Selachii. The cavum 

 prarcerebrale, which in the Selachii lies directly anterior to the 

 fenestra prarcerebralis, cannot be represented in any part of 

 the ethmoidal canal, for, aside from the origin of that canal as 

 above explained, the floor of the canal is not formed by the 

 trabecule, while the floor of the cavum prarcerebrale is. The 

 hind end of the ethmoidal canal is shown, in Schauinsland's 

 figure 165, lying but slightly anterior to the anterior surface of 

 the mid-brain. If this figure is correct, the membranous mesial 

 walls of the orbits must accordingly, even in the oldest embryos 

 considered by Schauinsland, be separated from each other by 

 a considerable interval. 



The chondrocranium of embryos of Callorhynchus thus ap- 

 parently owes the several points in which it differs from that of 

 the Selachii mainly, if not wholly, to the fact that that portion 

 of the central nervous system that lies anterior to the plica 

 encephali ventralis not only projects ventrally or antero-ventrally 

 at the time when the trabecular are laid down, but that, for some 

 reason, it has continued to lie in that position instead of later 

 gradually curving forward or forward and upward. The tra- 

 becular still apparently seek to curve upward into the line 

 prolonged of the parachordals, but, because of the interference of 

 the overlying brain, this upward curve is found immediately 

 anterior to the lobi olfactorii instead of, as in the Selachii, in the 

 pituitary region. 



The mandibular branchial bars were doubtless laid down 

 primarily at right angles to the trabecular, and hence, as in the 

 Selachii, in a nearly horizontal position, but as the trabecular 

 later grow downward and forward instead of, as in the Selachii, 

 curving gradually forward or forward and upward, the mandibular 

 bars have been distorted. The dorsal ends of the processus 

 oticus and basilaris, the latter representing the primitive dorsal 

 end of the arch, apparently remain approximately in their 

 primitive positions in relation, respectively, to the lateral wall of 

 the otic capsule and the trabecular, but they have been stretched 

 out into long cartilages by the marked ventro-anterior growth of 

 the trabecular, and the epal (quadrate) and ceratal (mandibuia) 

 elements of the arch lie in the region of the antorbital process. 

 This will be again referred to when describing the conditions in 

 Chimcera. Because of this distortion and change in position of 

 the mandibular branchial bars, the branchial bars of the more 

 posterior arches have also been carried forward and somewhat 

 downward without having been previously pushed backward to 

 the extent that they were in the Selachii, and the dorsal ends 

 of the anterior branchial bars are shown lying in the cranial 

 region in Schauinsland's oldest embryos. The sigma form of arch 

 lias, however, been impressed upon their dorsal ends, but not 

 upon their ventral ends. 



