ELASMOBRANCHII 



Sub-Class 1. ELASMOBRANCHII. 



The mouth is never quite terminal, a rostrum of considerable 

 size, supported by cartilaginous processes of the skull, being 

 generally produced in front. The two palato-quadrate cartilages 

 meet below the skull in front. The nostrils are more or less 

 ventral, in front of the mouth, and are incompletely subdivided 

 into anterior and posterior openings by a small flap (Figs. 114, 

 117). They lead into wide nasal cavities with two rows of olfactory 

 folds. A fronto-nasal process grows down between the nostrils in 

 the embryo, and either fuses with the upper lip so as completely to 

 separate the nostrils from the mouth (Fig. 114) or its edges form 



Fio. 84. 



Portion of the snout of Scyllium in section, showing ampullary tubes. (After Gegenbaur, 

 from Sedg wick's Zoology.) a, ampulla ; oi, passage of a tube through the dennis ; c, epidermis 

 c 1 , dennis ; n, nerve ; o, external openings of tubes ; t, tube. 



two deep grooves running from the mouth to the nostril on each 

 side (Fig. 103). The latter system, with 'confluent' nostrils, 

 appears to be the most primitive, and is found both in the Selachii 

 and in the Holocephali. The auditory vesicle often remains open 

 to the exterior by a narrow ductus endolymphaticus, even in the 

 adult (Fig. 13), and the sacculus contains a mass of small otoliths. 

 The eye, enclosed in a cartilaginous sclerotic, has a pigmented 

 tapetum, but the ventral ciliary process attached to the lens is 

 small ; it contains a muscle for accommodation (Fig. 346). 



The distribution of the lateral-line system on the head, fairly 

 constant among the Selachii, may here be described (Figs. 11, 85). 

 The main lateral line of the trunk runs forward on to the head, 

 where it may give off a transverse occipital branch ; continuing as 

 a short temporal canal and a postorbital canal, it divides into a 

 dorsal supraorbital and a ventral suborbital branch extending on 

 to the snout. Portions of a hyomandibular and mandibular canal 

 are also present, though generally interrupted (cp. p. 220, and Figs. 



