208 



FISHES 



CHAP. 



vertebra, and between them the remains of the notochord are 

 traceable into the cranial floor. In front of the occipital region 

 two lateral bulgings indicate the periotic capsules, arid more 

 anteriorly still, in the trabecular region, the sides of the cranium 

 are modified to form two spacious lateral recesses, the orbits, each of 

 which is bounded above and below by supra-orbital and infra-orbital 

 ridges respectively, behind by an outgrowth from the periotic 

 capsule (post-orbital process], and in front by a similar projection 

 from the hinder wall of the olfactory capsule (lateral etkmoidal 



Cr 



Nv.S 



ATv. 



'ex.br 



brr' 



ejz.br* 



Fia. 120. Side view of the skull of the common Dog-Fish (Scyllium canicula). aud.cp, 

 Auditory capsule ; br.a 1, 5, branchial arches ; br.r, br.r', cartilaginous rays 

 attached to the hyoid arch and the first four branchial arches ; Cr, cranium ; ex.br, 

 extra-branchial cartilages ; hy.cn, cerato-hyal ; hy.m, hyomandibular ; Ib, labial 

 cartilages ; Ig, ligaments passing from the jaws to the cranium and to the distal end 

 of the hyomandibular ; Ig', ethmo-palatine ligament ; l.j, lower jaw or Meckel's 

 cartilage ; Nv. 2, optic foramen ; Nv. 5, foramen for the Vth and part of the Vllth 

 cranial nerves ; olf.cp, olfactory capsule ; or, orbit ; up.j, upper jaw or palato- 

 quadrate cartilage. (From Wiedersheim, after W. K. Parker.) 



process}. In front of the cranial cavity and the orbits may be 

 seen the laterally-placed dome-like olfactory capsules, which are 

 open below, where the nasal sacs communicate with the exterior. 

 Between the two capsules an anterior extension of the cranial 

 floor forms a flattened mesethmoidal plate, behind which is the 

 large, membrane-closed, anterior cranial fontanelle. The lateral 

 walls of the cranium are perforated by numerous apertures, some 

 of which serve for the entrance or exit of blood-vessels, and 

 others, mostly pertaining to the inner walls of the orbits, for the 

 transmission of the different cranial nerves from the brain to 



