168 



SUB-CLASS GANOIDEI. 



segmented. The notochord is continued into the base of the 

 skull to the pituitary fossa in Polyodon, but not in Acipenser. 



In the Acipenseridae there are five rows of large scales with 

 projecting spines, * one dorso-median and two on each side 

 (Fig. 96). Between these are numerous small scale-like plates, 

 which frequently carry one or more spines. In the caudal 

 region these smaller plates, also carrying spines, are rhombic, 

 in contact, and regularly arranged. In Polyodon (Spatularia), 

 which is often described as naked, there are also small spine- 

 bearing scale-like structures. The scales are placed in the sub- 

 cutaneous tissue and are composed of bone (with bone corpuscles 

 -and branched processes, and in the larger scales haversian canals). 



The spines are composed of a 

 similar substance in layers, 

 either with bone cells in their 

 lower portions or without them. 

 There is no enamel and appar- 

 ently no ganoin. 



The cranium^ is massive and 

 largely cartilaginous, there being 

 but slight traces of ossification 

 (in the periotic region). As in 

 other Ganoids and in Teleostei 

 the inner wall of the otic capsule 

 is not developed. The roof con- 

 sists of almost continuous cartilage, but the fontanelles though 

 not conspicuous are not altogether absent. It is completely 

 covered by membrane bones which lie in the dermis. There 

 is a large parasphenoid and a single or paired vomer in the 

 floor. 



There is a partially ossified hyomandibular and a symplectic 

 cartilage (Fig. 95) which suspend the mandibular arch. The 

 anterior ends of the pterygoids meet in a symphysis. The 

 visceral arches are partially ossified but the meckelian cartilages 

 are unossified. The hyomandibular carries an opercular plate 

 and the hyoid arch a branchiostegal ray. Membrane bones (a 



* H. Klaatsch, Morph. Jahrb., 16, 1890, p. 146. O. Hertwig, ibid., 2, 

 1876, p. 373, 



f Bridge, Osteology of Polyodon. Phil. Trans. 1878, p. 683. 



FlQ. 04. Lcteral view of the vertebral 

 column of Spatularia (from Wieders- 

 heim). Cs sheath of notochord ; 

 Ic intercalated piece ; Ob neural 

 arch ; Ub haemal arch ; Ps neural 

 spine. 



