On the Anatomy and Phylogenetic Position of Polypterus. 401 



ledge is slight and unremarkable but the lower is larger and projects 

 considerably further. It is formed from the floor of the nose and 

 the lower thickening of the septum nasale. 



In one specimen there was a slit in the cartilage above the nasal 

 capsule on a level with the ectethmoid canal. It was not present in 

 the youngest specimen and must be considered as arising in Polypterus 

 first. The orbitosphenoid in front is replaced to a certain extent by 

 a downward process of the frontal. To its posterior upper angle runs 

 a prolongation of the parietal and the fusion of the two elements is 

 complete though in sections what is of parietal origin can be distin- 

 guished by pigment in the Haversian canals. The statement of Agassiz 

 that the orbitosphenoid of Folypterus is a descending process of the 

 frontal is a natural one though it is now known that its origin is double. 



The labyrinth region of the primordial cranium is of exceeding 

 interest and importance. Looking at it somewhat from behind 

 (fig. 13) one notices directly the remarkable vestibular protuberance 

 which is thin walled and remains long cartilaginous. Behind it the 

 Glossopharyngeus takes its exit. Above is the bone occupying the 

 external upper angle of the occiput. This bone may be called 

 the Petrosum. Traversing its external surface horizontally may be 

 seen a ridge which in front runs up to the articulation for the hyo- 

 mandibular. Where the ridge ends posteriorly is seen a small foramen 

 which transmits a dorsal branch of the Glossopharyngeus. The pe- 

 trosum above is prolonged to form the important projection which 

 supports the posttemporal scale and which has been variously called 

 epiotic, opisthotic or craniospinal process. Internally the petrosum sends 

 in that process which articulates with the exoccipital and in the 

 hollow below the two processes lies the large opening which trans- 

 mits not only the entire vagus nerve but also a large venous channel, 

 a part of the jugular. 



On the upper surface of the skull the backward process of the 

 squamoso-parietal enters into intimate relation with the tegmen cranii 

 the petrosum and the exoccipital. In old specimens the fusion of the 

 bones in this region is complete. 



In front of the vertebral elements which have been drawn into 

 (fig. 24) the base of the skull there occurred in the youngest si)ecimen 

 of Polypterus a small oval block of cartilage curiously surrounded by 

 a thin shell of bone and above it lay the threadlike termination of 

 the notochord. 



