SKULL— ELASMOBRANCHS 85 



tip of the rostrum, the anterior extension of the cranium. This 

 rostrum varies in form, sometimes being spoon-shaped, or by fenes- 

 tration of this, consisting of three bars which converge forwards. 

 It is sometimes so short as to be practically absent, but in skates 

 (fig. 88) it is very long, and in sawfishes extends most of the length 

 of the 'saw.' 



The parietal organ lies in a foramen, either in the membrane of 

 the fontanelle or in the cartilage a little farther back (fig. 87, ef). 

 The nasal capsules, with the nares on the ventral side, are joined 

 to the side cranial walls and open widely (olfactory foramina) to 

 the brain case. The orbits are bounded in front and behind by 

 pre- and postorbital crests which are connected dorsally by a strong 

 supraorbital crest, the fine between this and the wall of the brain 

 case being indicated on the dorsal surface by a row of foramina for 

 branches of the superficial ophthalmic nerve (fig. 87, os). Some 

 Elasmobranchs have a narrower suborbital crest, but this is usually 

 lacking, the orbits of the two sides being separated ventrally by a 

 median infraorbital ridge which usually bears near its middle on 

 either side a palatobasal articular surface (ba) over which the 

 palatal process of the lower jaw plays. The wall of the orbit is 

 perforated by vascular and nerve foramina, the position of which 

 varies with the genus. It also has connected with it a cartilage 

 optic pedicel which supports the eyeball. 



The otic capsules are less prominent than in the embryo. Ridges 

 on the surface of each show the position of the semicircular canals. 

 In the cranial roof between the capsules is a more or less developed 

 endolymph (parietal) fossa, in which are the openings of the endo- 

 and perilymph ducts. The base of the cranium has foramina for the 

 exit' of ninth and tenth nerves, the foramen magnum and smaller 

 openings for the spino-occipital nerves, the number of which varies 

 with the number of vertebrae included in the cranium. In skates 

 and Holocephah the foramen magnum is bounded laterally by occipi- 

 tal condyles for articulation with the first vertebra, but in sharks the 

 skull is immovably united with the spinal column. In Carcharias 

 the occipital region extends over the anterior vertebras. 



Most Elasmobranchs have seven visceral arches — mandibular, 

 hyoid and five branchial — but in Notidanids there are six {Hexanchus 

 and Chlamydoselachus — the latter sometimes with remnants of a 

 seventh) or seven (Heptanchus, fig. 69) gill arches. Most of the gill 



