VENTRAL SKULL 583 



upper jaw is by the hyomandibula (the upper element of the hyoid 

 arch) which is enlarged and bound by ligaments to the otic region 

 and to the quadrate. There may also be a slight connection of 

 jaw to cranium in the ethmoid region. In many primitive 

 Chondrichthyes, and in one modern group, the Notidani, the 

 suspension is called amphistylic ; the hyomandibula is used as 

 before, but there are also direct contacts of jaw and cranium in 

 the otic region and farther forward. The extreme tips of the jaws, 

 however, are free from the cranium, and articulate only with 

 each other. It is possible that this is the central primitive type 

 from which the others have been derived. 



The last type, autostyly, is found in the Holocephali, the 

 Dipnoi and the tetrapods ; the upper jaw is articulated (autodia- 

 styly or fused (autosystyly) to the cranium, and the hyomandibula 

 takes no part in its suspension. The condition in the Holocephali 

 has probably been achieved independently. In the tetrapods 

 there may be as many as four points where the palatoquadrate 

 bar meets the skull : at the extreme anterior end, in the ethmoid 

 region ; by an ascending process from the pterygoid part to 

 the wall of the orbit ; by a basal process just behind this, which 

 meets a basipterygoid process from the trabecular or polar 

 cartilage ; and posteriorly by an otic process to the auditory 

 capsule. Any of these may be lost or modified. The ethmoid con- 

 nection fails in amniotes and many urodeles ; the ascending 

 process of the pterygoid is absent from birds, but is strongly 

 developed in some reptiles and in mammals, where it grows right 

 into the side wall of the orbit, forming, as has been said, the 

 alisphenoid. The basal connection is prominent in amniotes, but is 

 absent from Anura and Apoda, while the otic process, though 

 always present, is in mammals quite detached from the jaws and 

 takes no part in the suspension (see p. 586). This condition, as 

 well as that of the Squamata and birds, where the quadrate, 

 though in contact with the cranium, is free to move, is called 

 streptostyly, in contradistinction to the monimostyly of crocodiles 

 and chelonians, where the quadrate is firmly fixed to the skull 

 and immovable. A further development of streptostyly, caUed 

 kinetism, is found in snakes and many birds, especially parrots ; 

 there is an articulation across the upper roofing bones, so that 

 the whole of the face and upper jaw can be raised when the 

 quadrate is swung forward. 



Below the palatoquadrate bar and its ossifications dermal 



