vii. 3 SKULL OF TELEOSTS 195 



bones has given much controversy. No generally acceptable theory 

 has yet appeared, perhaps because we know little of the factors that 

 cause separate bony elements to develop. There is some evidence of 

 relations between bones and teeth and bones and lateral line organs 

 and the pattern of the latter may play a large part in determining the 

 plan of the skull (p. 325). Provisionally we may recognize four classes 

 of dermal bones (1) canal bones, (2) tooth bones, (3) 'ordinary' bones, 

 whose determination is unknown, (4) extra bones, filling special areas 

 (Wormian bones). 



The arrangement of the numerous bones is made less difficult to 

 understand and remember if they are considered in the following 

 order. First the endochondral ossifications in the original neuro- 

 cranium, then the dermal bones that cover this above and below; next 

 the endochondral bones formed within the original cartilaginous jaws, 

 then the dermal bones that cover the edges of the jaws, and finally the 

 ossifications in the branchial arches and pectoral girdle, which latter 

 is in bony fishes attached to the skull. 



The endochondral ossifications may be considered by beginning at 

 the hind end of the skull: here the floor ossifies as the basioccipital, 

 the sides as the exoccipitals, and the roof, over the spinal cord, as the 

 supra-occipital bone; these posterior bones are not well marked off 

 from each other in the adult skull. In the auditory capsule are five 

 separate otic bones, of these the epiotic and pterotic can be seen 

 externally (Fig. 118). 



The floor in front of the basioccipital is occupied by a basisphenoid 

 bone and the walls above this by alisphenoids. The eyes nearly meet in 

 the midline and the orbits are here separated only by a thin orbito- 

 sphenoid. The only more anterior part of the chondrocranium to 

 ossify is the region between the nasal capsule and the orbit, forming 

 the ectethmoid. 



The dermal bones that cover this partly ossified neurocranium may 

 be identified as on top and in front a pair of frontals and a median 

 supraethmoid, behind which are large paired parietals and small 

 paired post-parietals. These names have been inferred from study of 

 crossopterygians and early amphibians (p. 325), which showed that 

 the homologies earlier accepted were wrong. Fig. 118 carries the old 

 nomenclature in which the large paired bones were called frontals. 

 Around the eyes is a ring of circum-orbitals, and on the floor of the 

 skull two median bones, the parasphenoid and vomer. 



The jaw bones are numerous, including both endochondral and 

 dermal elements, and the relation of the method of support to that 



