GREGORY: FISH SKULLS 



115 



Meckel's cartilage, the proximal end of which was ossified to form a large articular. A 

 row of coronoid bones supported small denticles; the dentary was the principal jaw bone and 

 supported the large teeth. The angular was of large size and seems to correspond with the 

 dermarticular of typical fishes. The prearticular was a large flat plate. A conspicuous 

 difference from the crossopterygian-tetrapod type was the absence of a separate row of 

 infradentaries. The branchiostegal pieces — doubtless attached to the cerato- and epi- 

 hyals — were wide and flat, the branchiostegal-opercular series not being interrupted by 

 any obliquely placed interopercular. Thus the upper jaws and mandible of primitive 

 palaeoniscids as well as the surface pattern of the skull appear to be practically prototypal 

 to that of higher ganoids and teleosts. 



semicircular canah 



notochorcL 



Fig. 14. Median sagittal section of ncurocranium of (A) CManydoselachus (after .Allis) and (B) palsoniscid (after 

 Watson) 



The ncurocranium of various Palaeozoic and later chondrosteans has been described by 

 Stensio in 1921 and 1925 and by Watson in 1925 and 1928. The ncurocranium of the 

 palseoniscids differs from the braincase of typical sharks, especially in the following points 

 (Figs. 13, 14): (1) the nearly complete ossification of the braincase; (2) and the presence of 

 separate ossific centres (at least in one genus) for the opisthotic, basioccipital and sphenotic; 

 (3) the functional integration of the deep and surface ossifications; (4) the presence and 

 importance of the parasphenoid or keel bone; (5) the reduction of the ventral part of the 

 interorbital braincase to a thin septum; (6) the elevation of the interorbital brain trough 

 above the parasphenoid; (7) the development of a myodome; (8) the presence of canals 

 in the base of the cranium for the dorsal aorta and the eflferent branchial arteries, as well 

 as for the external and internal carotids. 



Not improbably the parasphenoid bone may at first have been developed in order to 

 stiffen the floor of the orbit, especially as the recti muscles extended backward in the 

 manner described by Watson (1925, p. 849). 



Many detailed differences seen between the palaeoniscid and the shark in the interior 

 4 



