forerunners of dermal bones. The labial cartilages appear 

 to be comparable in their origin to the maxillary and den- 

 tary of other fish. The prismatic calcified cartilage of 

 elasmobranchs is not primitive but clearly a highly modified 

 hard tissue which cannot possibly be viewed as the forerun- 

 ner of the enchondral bone observed in other fishes. From 

 this it can be concluded that sharks may never have been 

 extensively bony, either dermally or chondrally. 



Holocepbalan or chimaerid The living holocephalans be- 

 long to five genera: Chimaeta, Hydrolagus (Figures 5-18 to 

 5-20), Callorhynckus, Harrwta, and Rhinochimaera. Fossils of 

 this group extend back to the Upper Triassic (Squaloraja 

 and Myriacanthus). It has been assumed on the basis of the 

 tubulodentine of the tooth plates (Figure 8-55) that these 

 fishes are derived from the Bradyodont sharks dating back 

 to the Upper Devonian. 



Comparisons of the shark and chimaerid suggest that 

 they have little in common. The differences in terms of 

 Hydrolagus can be summarized as follows: 



1. Palatoquadrate fused to the neurocranium from olfac- 



tory capsules to otic capsule (holostylic style of jaw 

 suspension). 



2. "Hyomandibula" only a slightly modified epihyal which 

 does not articulate with the cranium; there is a separate pos- 

 teromedially projecting pharyngohyal; there is no spiracle 

 nor spiracular cartilages; there is a single external opercular 

 opening on either side. 



3. Precerebral fossa (or precerebral fontanelle) and capsu- 

 lar canals are lacking; nasal capsules are separated by nasal 

 septum. 



4. Posterior cerebral vein exists independently of the vagus. 



5. Occipital condyles better marked. 



6. Otic capsule lacks inner wall; a midline endolymphatic 

 foramen opens through the cranial roof. 



7. Ethmoid chamber present for myeloid tissue and passage 

 of superior ophthalmic and profundus nerves. 



8. Optic pedicel lacking. 



9. Rostral cartilage present which supports spongy tissue of 

 snout. 



There is some agreement with the shark in features such 

 as lack of bony jaws, chondrification of the notochordal 



occipitospjnal nerve foramina 



nasal cartilages 



pedicular 

 cartilage 



upper tooth plate 

 lower tooth plate 



fibrous mass in lower lip 

 labial cartilage 

 Meckel's cartilage 



pectoral girdle 



Figure 5-1 8. Head skeleton of Hydrolagus colliei 

 orbital foramina; C, labels for occipital foramina. 



A, lateral view of entire area; B, details of 



GNATHOSTOME FISHES • 123 



