THE SKULL OF CHJ3LKRA. 



121 



process, that point marking also the anterior end of the ethmoidal 

 canal. This latter canal is large and is separated from the 

 cranial cavity by cartilage slightly thinner than that of the 

 basis cranii. Beyond the anterior end of the ethmoidal canal 

 the cranial cavity contracts .somewhat abruptly to its anterior 

 end, that end being marked, on either side, by the foramen 

 olfactoriom. The cranial cavity of the adult accordingly shows, 

 when compared with Dean's figures of embryos of this fish and 

 with Sehauinsland's figures of embryos of Callorhynchus, that 

 the dorsal portion of the chondrocranium has undergone marked 

 lateral compression in post-embryonic stages, the membranous 

 mesial walls of the orbits being pressed together so that they 

 meet and coalesce in the median plane in front of the midbrain 

 and dorsal to the forebrain, a considerable portion of the cranial 

 cavity of embryos thus being obliterated. Because of this, or 

 correlated to it, the midbrain becomes, in the adult, widely 

 separated from the hind end of the ethmoidal canal, and the 

 forebrain is correspondingly elongated. 



The two foramina olfactoria are separated from each other by 

 a stout column of cartilage which lies nearly at light angles to 

 the cranial floor, and each foramen is closed by a membranous 

 cribriform plate. Each foramen opens directly into the mesial 

 portion of the posterior end of the related nasal capsule, that 

 capsule being directed antero-ventrally and bulging laterally so 

 that it forms a pronounced swelling on the lateral surface of the 

 chondrocranium. The ventral edge of the capsule reaches nearly, 

 but not quite, to the ventral edge of this part of the chondro- 

 cranium, and the capsules of opposite sides are in contact in the 

 median line, a septum nasi separating them from each other. 

 This septum nasi is continuous, posteriorly, with the column 

 of ca.rtilage that separates the foramina olfactoria, that column 

 lying in large part posterior to the plane of the foramina and 

 representing the rostral stalk of embryos of Callorhynchus and 

 of the Selachii. The septum nasi thus lies morphologically ventral 

 to the stalk, and is formed either by a subethmoidal keel alone, or 

 by that keel fused with the contiguous mesial sides of the nasal 

 capsules, the rostral stalk forming the morphologically dorsal 

 edge of the septum, as it also does in Mustelus (Gegenbaur, 

 1872, fig. 1, pi. 5) and Scyllium (Parker, 1876, fig. 4, pi. 37). 

 The nasal capsules thus fuse with each other, in Chimcera, ventral 

 to the trabecular, as they do in the Selachii, this confirming the 

 conclusion already arrived at that they have this position in 

 Callorhynchus. 



With the contact and fusion of the nasal capsules in the 

 median line ventral to the trabecular, the ventro-lateral trabecular 

 processes of opposite sides, if they existed in this fish, must also 

 have been pressed together in the median plane, and their flaring 

 ventral ends probably form some part of the beak-like prenasal 

 process, that process being, as already stated, morphologically 



Proc. Zool. Soc— 1917, No. IX. 9 



