FISH SKULLS: A STUDY OF THE EVOLUTION OF 

 NATURAL MECHANISMS 



THE BEGINNINGS OF THE FISH SKULL 



The typical fish skull, or syncranium (Fig. 1), notwithstanding the intricacy of its 

 details, is generally recognized to be composed of two sharply contrasting divisions, which 

 may be called the neurocranium, or braincase, and the branchiocranium. The main parts 

 of the neurocranium are: first, a series of inner, or endosteal, elements that surround and 

 protect the olfactory, optic and otic capsules and the anterior end of the notochord; second, 

 a series of superficial ectosteal, or derm bones. These derm bones were originally similar 

 to ganoid scales in microscopic structure but have long since lost their enamel-like surface. 

 Some of them have become pitted, tunneled, or inflated by the lateral line organs which 

 pass over or through them. They are also modified in various ways by the deeper layers 

 of the skin which now often covers them. The adult teleost endocranium may also be 

 considered as a complex of four intergrading parts surrounding the orbits; these may be 

 named the ethmovomer block, the interorbital bridge, the cranial vault and the keel bone 

 or parasphenoid.^ 



Fig. 1. The syncranium and its parts. Diagram showing sagittal section of the neurocranium and inner aspect of right 

 half of branchiocranium in a typical teleost fish (Roccus tineatus). 



The branchiocranium includes the mandibular region (comprising the oromandibular 

 arch and attached derm bones), the hyal region (the hyoid arch and opercular series) and 

 the branchial arches with their attached dermal plates. 



The modern fish skull is subdivided into a large number of separate bones, each one 



' For a discussion of the functions of these four parts, see p. 434. 



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