THE SKULL. 581 



and forms what may be called the pterygo-quadrate cartilage, 

 though not completely homologous with the similarly^ named 

 cartilage in Elasmobranchs ; while the lower region forms the 

 Meckelian cartilage (Mck], which has already grown inwards, so 

 as to meet its fellow ventrally below the mouth. The whole 

 arch becomes at the same time widely separated from the axial 

 parts of the skull. 



Nearly simultaneously with the first differentiation of the 

 mandibular arch, a bar of cartilage the palatine bar already 

 spoken of is formed on each side, below the eye, in front of the 

 mouth. The dilated anterior extremity of this bar soon comes 

 in contact with an anterior process of the trabeculae, known as 

 the ethmopalatine process. 



In a later stage the pterygoid end of the pterygo-quadrate 

 cartilage unites with the distal end of the palatine bar (fig. 334, 

 Pl.Pt), and there is then formed a continuous cartilaginous 

 arcade for the upper jaw, which is strikingly similar to the 

 cartilaginous upper jaw of Elasmobranchii. 



A large dorsal process of the primitive pterygo-quadrate now 

 forms a large metapterygoid tract (M.Pf) ; while the whole arch 

 becomes firmly bound to the hyomandibular (H.M}. 



In the later stages the parts formed in cartilage become 

 ossified (fig. 335). The palatine is first ossified, the pterygoid 

 region of the pterygo-quadrate is next ossified as a dorsal 

 mesopterygoid (m.pg) and a ventral pterygoid proper (/"). 

 The quadrate region, articulating with the Meckelian cartilage, 

 becomes ossified as a distinct quadrate (qu}, while the dorsal 

 region becomes also ossified as a metapterygoid (int.pg). 



In the Meckelian cartilage a superficial ossification of the 

 ventral edge and inner surface forms an articulare (ar) ; but the 

 greater part of the cartilage persists through life. 



Some of the above ossifications, at any rate those of the palatine and 

 pterygoid, seem to be started by dental osseous plates adjoining the carti- 

 lage. They will be spoken of further in the section dealing with, the mem- 

 brane bones. 



Amphibia. The development of the autostylic piscine skulls 

 has unfortunately not yet been studied ; and the most primitive 

 autostylic types whose development we are acquainted with are 



