io6 



VERTEBRATE SKELETON 



genera passing through it, behind it in others. Basicranial fenestrae, anterior 

 and posterior, may occur nearer the median Hne, the anterior often being 

 confluent with the hypophysial fenestra, the posterior partly or wholly divided 

 into riglit and left halves by the notochord. The walls of the posterior part of 

 the chondrocranium are formed by the occipital vertebrae and the otic capsules, 

 the latter widely open on the medial sides. In the interorbital region of platy- 

 basic crania there is no constriction, the walls being formed by trabeculae and 

 sphenolaterals. In tropibasic skulls an interorbital septum extends upwards 

 from the common trabecula, this when high, restricts the brain to the postorbital 

 parts. In all species a narrow marginal band (sphenolateral) of cartilage 



extends, dorsal to the orbit, from otic 

 capsule to the roof (tegmen) of the 

 anterior part of the skull. This 

 anterior tegmen is wide in some fishes, 

 narrow in others where it lies in or 

 behind the plane of the pineal organ. 

 There is always a posterior fontanelle, 

 an anterior also when the anterior 

 tegmen is narrow. The posterior 

 fontanelle is Umited behind by the 

 synotic tectum and is often divided 

 into right and left halves by a nar- 

 row cartilage bar extending from the 

 tegmen towards or to the synotic 

 tectum, a remnant of the complete 

 Elasmobranch roof. 



The nasal organs are separated 

 by a median nasal septum, a con- 

 tinuation of that in the interorbital 

 region. The floor of each capsule is 

 formed by the lateral part of the eth- 

 moid plate, the roof is an expansion 

 of the tegmen. Behind, the capsule 

 is limited by a cartilage, the homo- 

 logue of the antorbital crest of sharks. 

 In Siluroids, Cyprinoids and Gadids 

 the olfactory nerve follows the normal course inside the cranial cavity from 

 brain to olfactory organ. In many other Teleosts, as a result of the interorbital 

 septum, the nerve enters the orbit and then reaches the nasal cavity by passing 

 through the antorbital crest. 



The visceral skeleton arises much as in Elasmobranchs, except that it is 

 largely ossified in the adult. The pterygoquadrate has its pterygoid part 

 joining the ethmoid by one or two processes, and associated with it may be a few 

 isolated cartilages (a subrostral in Salmo, submaxillary in Catostomus and 

 Perca, and one at the angle of the mouth in many genera). These are regarded 

 as remnants of labial cartilages (p. 68). The quadrate part of the pterygo- 



FiG. 113. — Three successive stages in the 

 development of the chondrocranium of 

 Salmo fario (Woskoboynikow in Schinip- 

 kewitsch, '21). First stage black; second 

 stippled; third shaded, ct, trabecular cornu; 

 e, ethmoid plate; h, hypophysis; w, noto- 

 chord; oc, otic capsule; pc, parachordal; ps, 

 sphenolateral; st, synotic tectum; /, tra- 

 becula; Ic, tegmen cranii. 



