. The elasmobranch fishes . Fig. 65. Dorsal view of cranium, Zygaena, left side. (Modified from Gegenbaur.) or., orbit; , postorbital process; , preorbital process. In a side view (figs. 66 and 68) the auditory, optic, and olfactory regions of the cranium are seen to advantage. With the exception of ChJamydosclachiis and the notidanids the auditory region is greatly modified superficially for the attachment of the hyoid arch. In most of the recent sharks the articulation is made by means of a deep pit (as is present in Heterodontus, fig. 66). In some of the rays a special part from t
. The elasmobranch fishes . Fig. 65. Dorsal view of cranium, Zygaena, left side. (Modified from Gegenbaur.) or., orbit; , postorbital process; , preorbital process. In a side view (figs. 66 and 68) the auditory, optic, and olfactory regions of the cranium are seen to advantage. With the exception of ChJamydosclachiis and the notidanids the auditory region is greatly modified superficially for the attachment of the hyoid arch. In most of the recent sharks the articulation is made by means of a deep pit (as is present in Heterodontus, fig. 66). In some of the rays a special part from the hyoid arch makes an extended articulation with this part of the cranium. Anterior to the auditory region is the enlarged orbit in which the eyeball rests. Its roof is usually formed by a supraorbital crest, modified posteriorly into a postorbital process {), and anteriorly into a preorbital process {). In the sharks the postorbital process is rarely extended. Exception must be made, however, of Zygaena, the hammerhead {, fig. 65) and its near ally, the bonnet shark, where it may be prolonged outward to meet the posterior part of the preorbital process. Exception should be made also of Chlamydosel- achus and the notidanids in which the postorbital process serves in suspend- ing the upper jaw. In the rays, except those bearing stings, this process is characteristically small or absent. The preorbital process in Zygaena {, fig. 65) arises far out on the cra- nium and is divided into anterior and posterior jiarts. The anterior part
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