58 



THE ELASMOBRANCH FISHES 



111 the olfactory region the capsules are seen in their relation to the elliptical 

 nasal cartilages (w.c, figs. 60. 63, and 64) surrounding the nasal aperture. 

 Projections from the anterior and posterior margins of the ellipse meet and 

 cross, usually forming of it a figure 8. A second anterior accessory process may 

 be added as in Heterodontus {n.c.~, fig. 60), or may be more completely de- 

 veloped as in Scyllium. In the rays the ellipse may be broken into segments. In 

 Rhinohatis (fig. 64) the anterior and posterior projections forming the bridge 

 are relativelj^ slender cartilages. In Myliohatis numerous accessor}^ projec- 

 tions are present. 



Fig. 65. Dorsal view of cranium, Zygaena, left side. (Modified from Gegenbaur.) 

 or., orbit; po.o., postorbital process; pr.o., 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 {po.o.), and anteriorly into a preorbital process 

 {pr.o.). 



In the sharks the postorbital process is rarely extended. Exception must be 

 made, however, of Zygaena, the hammerhead {po.o., 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 {pr.o., fig. 65) arises far out on the cra- 

 nium and is divided into anterior and posterior jiarts. The anterior part 



