644 J. PLAYFAIR MCMURRICH. 



Marshall's observations are apparently conclusive as to the 

 segmental nature of the olfactory nerve, and the homology of 

 the Schneiderian membrane of the olfactory capsules with the 

 gills of the posterior arches. Dohru's ^ researches on the 

 pituitary body of fishes tends to refer this structure also to a 

 visceral cleft, and there is a union between it and the nasal 

 cavity in Petromyzon, which, however, Dohrn states is 

 merely secondary. We have here, then, a nerve, and probably 

 a cleft, but the remaining structures are apparently absent. 

 As to the head-cavities in Elasmobranchs, there is only one 

 prsemandibular ; and in Teleosts Ganin states ^ that this is the 

 only one present, evidently showing that there is a tendency 

 for these structures to disappear; and we may conjecture that 

 there was originally a second prsemandibular cavity, which has 

 disappeared even in the Elasmobranchs. 



The visceral arch corresponding to this segment, I am in- 

 clined to think, is represented by the trabeculse cranii. The 

 lowest type of Vertebrate presents no prolongation of the 

 cerebral nervous system beyond the extremity of the notochord. 

 The portion in the higher vertebrates anterior to that point is 

 merely an overgrowth, and we must consider the pituitary 

 region as corresponding to the extremity of the under-surface 

 of the brain in Amphioxus. Accordingly we have a pree- 

 ver'iebral portion of the cranium which is supported by the 

 coalesced trabeculse cranii. These structures at first are two 

 cartilaginous bars, articulating with the extremities of the 

 parachordals, and extending down parallel to the posterior 

 visceral arches. Eventually they become bent upwards so as 

 to run parallel with the long axis of the skull, unite anteriorly, 

 and finally send up lateral outgrowths to form the side wall of 

 the anterior region of the skull. At first they present no 

 differences in appearance or relations from the visceral arches. 

 They are curved so as to approximate below, and articulate 

 with the vertebral region of the skull. This represents the 



' A. Dohrn, loc. cit. See also ' Zool. Anz.,' vol. v, 1SS2. 

 ^ Ganin, " Ucb. die Eutvv. des Kopfskclcts bei KuockcnGschc (Rhodcns 

 Gaaterostens)," 'Zool. Anz.,' No. 51. 



