334 PEOF. T. W. BEIDGE ON THE MOEPHOLOGT OF THE SKULL 



of the central portion of the cranial cavity has already been mentioned (see fig. 4). 

 Reference to transverse sections of the skull (figs. 16 and 17) will show that in 

 this region each lateral margin of the parasphenoid divides into an ascending lamina 

 which meets the descending portion of the fronto-parietal internally to the trabecular 

 cartilage, and a deflected lamina which, as mentioned above, overlaps the inner surface 

 of the palato-pterygoid bone of its side. These sections also prove that in the same 

 region the parasphenoid alone forms the floor of the cranial cavity, the cartilaginous 

 basis cranii being here entirely absent. Perhaps the two most remarkable features 

 in connection with the parasphenoid of Lepidosiren are the share which, with the 

 fronto-parietal, it takes in forming the lateral walls of the cranial cavity, and the 

 somewhat abrupt termination of the bone anteriorly, so that a considerable area of the 

 cartilaginous basis cranii is freely exposed between its anterior margin and the palato- 

 pterygoid symphysis. 



The bone termed " palatine " by Bischoff and Hyrtl in Lepidosiren — in Protopterus 

 considered as equivalent to the maxilla, pterygoid and palatine (Owen), or termed 

 " Gaumenkieferbein " (Peters), " palatine " (Cobbold), " Pterygo-palatinum " or 

 " palato-pterygoid " (Wiedersheim and Huxley) ; and in Ceratodus " pterygo-palatine " 

 (Giinther, Huxley) — is one of the most characteristic elements in the Dipnoid skull 

 (PI. XXVIII. figs. 1-4 ; PL XXIX. figs. 12-17, p.pt.). The bone seems to represent 

 a pterygoid element ossified continuously with a " tooth-bone " formed by the fusion of 

 the strongly-developed palatal teeth, and may therefore rightly be regarded as a 

 palato-pterygoid. 



Posteriorly the bone makes its appearance at a point immediately anterior to the 

 foramen for the exit of the hyomandibular division ot the Facial nerve in the form of 

 a relatively wide thin plate, closely applied to the inner surface of the suspensorial 

 cartilage, and overlapped along its inner and dorsal margin by the deflected lateral 

 edge of the parasphenoid (fig. 3). Traced downward towards the articular extremity 

 of the suspensorial cartilage the bone thickens somewhat, and forms the inner margin 

 of the articular surface for the lower jaw (figs. 3 and 4). From this point the palato- 

 pterygoid loses its lamellar character and becomes an exceptionally strong, laterally- 

 compressed bone, arching upward and forward towards the nasal capsule. In the latter 

 part of its extent it is in relation by its dorsal margin with the band-like trabecular 

 cartilage (fig. 1), and by it is separated from the sutural union of the descending lamina 

 of the fronto-parietal and the parasphenoid (fig. 16). In front of the anterior termi- 

 nation of the latter bone the palato-pterygoid skirts the lateral margin of the cartila- 

 ginous basis cranii (fig. 15). On reaching the hinder boundary of the nasal capsule, 

 each palato-pterygoid begins to curve obliquely inward and forward, and is finally united 

 with its fellow in an elongated and exceptionally massive symphysis which is interposed 

 between the two nasal capsules, and dorsally is in contact with their roof in the median 

 plane (figs. 3, 4, and 12). The extraordinary development of the symphysial portions 



