112 



MR. E. PHELPS ALLIS, J UN., ON 



adult Mustelus, lying posterior to the foramen olfactorium and 

 directly above the lobus olfactorius. 



The dome-shaped protuberance of Callorhynchus, the so-called 

 nasal capsule, always lies, at all stages of its development, in the 

 region between the bases of the median and lateral rostral 

 processes, and as the capsule increases in size the bases of the 

 rostral processes are correspondingly separated from each other, 

 and the anterior opening of the cranial cavity carried corre- 

 spondingly forward ; but whether this growth of the cranial wall 

 takes place posterior to the fenestra olfactoria of the 60 mm. 

 embryo, or is due to growth anterior to that fenestra, cannot be 

 told from the figures. In the one case the nasal sac, which 

 certainly lay primarily anterior and external to the cranial 

 cavity, would have been pulled relatively backward into the 

 anterior end of that cavity; the fenestra olfactoria of the 60 mm. 

 embryo would remain morphologically unchanged, but would 

 change in function from a fenestra olfactoria to a fenestra 

 nasalis ; and the so-called nasal cartilages of Schauinsland's 

 figures would represent detached pieces of an undevelopeel nasal 

 capsule. In the other case, the nasal capsule would be developed 

 its a, direct anterior, but morphologically ventral, prolongation of 

 the cranial w r alls, a fenestra, nasalis gradually developing external 

 to the fenestra olfactoria of the 60 mm. embryo. The conditions 

 in the adult Chimcera decidedly favour this latter view, but one 

 of Schauinsland's figures of Callorhyiichus ecjually decidedly 

 favours the former view. The figure in question (fig. 165) gives 

 a, median vertical sectional view of an embryo older than the 

 85 mm. one, and shows the brain in place in the cranial cavity. 

 The trabecule are shown lying in the line pre)longed of the 

 parachordals, which is not in accord with the figures of other 

 embryos both older anel younger than this one. The septal 

 cartilage is shown as a simple rostral stalk, while in the younger, 

 85 mm. embryo, it is already an important plate-like structure 

 the posterior portion of which corresponds to the rostral stalk 

 of the 60 mm. embryo, and the anterior portion to the sub- 

 ethmoidal keel of that stalk. The rostral stalk of the embryo 

 shown in figure 165 projects clorso-posteriorly, instead of, as in 

 the other embryos, dorso-anteriorly, and the ventral portion 

 of the nasal sac is shown extending forward anterior to the level 

 of the base of the median rostral process and close to the ventro- 

 anterior corner of the ehondrocranium. The nasal sacs, as here 

 shown, must accordingly project beyond the anterior openings of 

 the cranial cavity, if those openings are found in this embryo at 

 all in the positions that they have in the other embryos, both 

 older and younger, anel furthermore, they must lie dorsal to the 

 trabecular, as they do in Ceratodus and the Teleostei, instead of 

 ventral to the trabecular, as they clo in the Selachii and as they 

 must also in Callorhynchus if the septum nasi of this fish lies 

 ventral to the rostral stalk, as shown in Schauinsland's other 



