THE SKELETON IN FISHES. THE SKULL. 119 



orbital process behind it. The nasal capsules are always carti- 

 laginous, and the eye, as a general rule, has a cartilaginous 

 sclerotic investment. The cranium is often prolonged in 

 front into a rostrum which is enormously developed in Pristis 

 and some Rays. The cartilaginous roof of the cranium is 

 rendered incomplete by the presence of a large hole, the an- 

 terior fontanelle. 



Two pairs of labial cartilages (fig. 16, B, 8) are often 

 present. They lie imbedded in the cheeks outside the anterior 

 region of the jaws, and are specially large in Squatina. 



As regards the visceral arches 1 the simplest and most primi- 

 tive condition of the jaws is that of the Notidanidae, in which 

 the niandibular and hyoid arches are entirely separate. In 

 these primitive fishes the palato-pterygo-quadrate bar articu- 

 lates with the postorbital process (fig. 16, 10), while further 

 forwards it is united to the cranium by the ethmo-palatine 

 ligament. The hyoid arch is small and is broadly overlapped by 

 the niandibular arch. The term autostylic is used to describe 

 this condition of the suspensorium. From this condition we 

 pass in the one direction to that of Cestracion (fig. 16, B), in 

 which the whole of the palato-pterygo-quadrate bar has become 

 bound to the cranium, and in the other to that of Scyllium. 

 In Scyllium (fig. 6), while the ethmo-palatine ligament is 

 retained, the postorbital articulation of the palato-pterygo- 

 quadrate has been given up, so that the palato-pterygo-quadrate 

 comes to abut on the hyomandibular and is attached to it by 

 ligaments. The pre-spiracular ligament (fig. 16, 20) running 

 from the auditory capsule also assists in supporting the jaws. 



Lastly we come to the purely hyostylic condition met 

 with in Rays, in which the niandibular arch is entirely sup- 

 ported by the hyomandibular. In some Rays the hyoid is 

 attached to the posterior face of the hyomandibular near its 

 proximal end, and may even come to articulate with the 

 cranium. 



1 See H. B. Pollard, Anat. Am. x. 1894. 



