1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 31 



filaments, the anterior cerebral vesicle is already distinctly divided 

 into the thalamencephalon (from which the large infundibulum pro- 

 ceeds below, and the small tubular peduncle of the pineal gland above, 

 while the optic nerves leave its sides) and a large single oval " vesicle 

 of the hemispheres." On the ventral face of the integument covering 

 these are two oval depressions, the rudimentary olfactory sacs. 



As development proceeds, the vesicle of the hemispheres becomes 

 divided by the ingrowth of a median longitudinal septum, and the 

 olfactory lobes grow out from the posterior lateral regions of each 

 " ventricle " thus formed, and eventually rise onto the dorsal faces 

 of the hemispheres, instead of, as in most Vertebrata, remaining on 

 their ventral sides. I may remark that I cannot accept the views of 

 Miklucho-Maclay, whose proposal to alter the nomenclature of the 

 parts of the Elasmobranch's brain appears to me to be based upon 

 a misinterpretation of the facts of development. 



II. The Skull. 



Dr. Giinther * distinguishes in the skull an " inner cartilaginous 

 capsule and an outer incomplete osseous case, to wliich again some 

 outer cartilaginous elements are appended. In the former the con- 

 Fig. 4. 



Ceratodus forsteri. Lateral view of the chondrocranium, with most of the 

 bones and fibrous tissues removed. 

 ol, position of the olfactory .sac ; an, pi, position of the antisrior and posterior 

 nares; a, process of the cranial cartilage, d, between the two; 1, 2, upper 

 labial cartilages ; op, optic foramen ; A. 0, antorbital process ; P. 0, post- 

 orbital process ; P. Q, palato-quadrate process ; Sq, Qu, remains of the bony 

 plate which lies on the outer side of this process; O.C, occipital prolon- 

 gation of the chondrocranium roofing over the branchial cavity ; I), den- 

 tary; ^«, angular; ^, hyoid ; I.op, interoperculum ; Op, operculum ; R, 

 B', cartilages representing hyoidean rays ; HM, hyo-mandibular. 



fluence of cartilage is so complete, that no distinct divisions are trace- 

 able by sutures ; its parts can be designated only by reference to the 

 locally corresponding bones of the teleosteous skujl." 



» L. V. p. 521. 



