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TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



posterior axis, the slowest along the vertical axis, while in Psettus the reverse is the case 

 (Fig. 297). Moreover, in watching the development of any fish we see that the changes in 

 form take place through differential growth rates affecting different regions at different 

 periods of development. Hence we arrive at once at the significant if obvious concept that 

 the various parts of the skull share in general regional or extrinsic growth rates as well as 

 having individual or intrinsic growth tendencies of their own. For example, the circum- 

 orbital bones around the eye always fit into the space between the eye and the rim of the 

 preopercular. If the distance between the eye and the preopercular increases, several of 

 the bones behind the eye may share the increased length, as in Arapaima (Fig. 59), or only 

 one of them may do so, e.g. the suborbital stay of scorpaenoids (Fig. 201). Here we have 

 a good example of the fact that in general each part has its own intrinsic growth rate, plus 

 the extrinsic factor which it derives from other parts of the body. 



Hence, to consider the neurocranium itself, it is now obvious that In a given type of 

 fish the precise form and details of the neurocranium will depend upon the resultant of a 

 multitude of regional (extrinsic) and local (intrinsic) Influences; nevertheless, with the 

 experience gained from a detailed study of a large number of different skull types it seems 

 possible to distinguish a few of the more conspicuous evolutionary factors in any given skull. 



Functions of the Four Main Parts of the Neurocranium 



In a general way the neurocranium (Fig. 298) in the side view is typically more or less 

 like a right triangle, its base being the vomer-parasphenoid-basioccipital. Its hypothenuse 



ptofst) 



pf * pareth 



Lates TiilotLcus 



Fig. 298. Syncranium of a typical percoid (Lates niloticus). 



