XX] SKULL 457 



connected with the cranium either directly, by articulation with the 

 cranium in front of the auditory region, an arrangement 'called 

 autostylic and prevailing among recent Elasmobranchs only in 

 the Notidariidae ; or else- the upper jaw has lost its articulating 

 process with the cranium and is instead firmly connected or slung 

 by ligament into the hyomandibular, which thus suspends the jaw 

 from the skull. This arrangement, called hyostylic, is that seen in 

 the majority of Elasmobranchs. A modification, termed amphi- 

 stylic, occurs in Heterodontidae, where the jaw is slung by the 

 hyomandibular but also has acquired direct articulation with the 

 skull in front of the orbit. The remaining visceral arches have only 



FIG. 223. Lateral view of the skull of a Dog-fish (ScylUum caniculi) x -. From 



Reynolds. 



1. Nasal capsule. 2. Rostrum. 3. Interorbital canal for the passage of 

 a blood-vessel. 4. Foramen for hyoidean artery. 5. Foramen for the 

 exit of the ophthalmic branches of Vth and Vllth nerves. 6. Foramen 

 through which the external carotid leaves the orbit. 7. Orbitonasal 

 foramen which allows a blood-vessel to reach the nose. 8. Auditory 

 capsule. 9. Foramen through which the external carotid enters the 

 orbit. 10. Ethmopalatine ligament. 11. Palatopterygoquadrate bar. 

 12. Meckel's cartilage. 13. Hyomandibular. 14. Ceratohyal. 



15. Pharyngobranchial. 16. Epibranchial. 17. Ceratobranchial. 

 18. Gill-rays; nearly all have been cut off short for the sake of clear- 

 ness. 19. Extrabranchial. n, in, iv, v, va, vna, ix, x. Foramina 

 for cranial nerves. 



a muscular connection with the skull and are termed the branchial 

 arches, since to their sides are attached the gills. The branchial 

 arches are jointed into several pieces, which are placed in an oblique 

 position and so arranged that when they are raised by the levatores 

 arcuum muscles attaching them to the skull they diverge and 

 expand the gill-sacs lying between them. The segments of each 

 branchial arch are typically four in number, named respectively 

 pharyngobranchial, epibranchial, ceratobranchial, and 

 hypobranchial. The first-named are situated in the dorsal wall 



