478 MAXDIBULAK AND HYOID BARS. 



the arch do not meet anteriorly, but are separated by the ends of the 

 trabeculae. 



The anterior bar of the upper arcade is known as the palatine ; 

 but it appears to me as yet uncertain how far it is to be regarded 

 as an element, primitively belonging to the upper arcade of the 

 mandibular arch, which has become secondarily independent in its 

 development ; or as an entirely distinct structure which has no 

 counterpart in the Elasmobranch upper jaw. The latter view is 

 adopted by Parker and Bridge, and a cartilage attached to the 

 hinder wall of the nasal capsule of many Elasmobranchii is identified 

 by them with the palatine rod of the Teleostei. 



The arch itself is at first very similar to the succeeding arches ; 

 its dorsal extremity soon however becomes broadened, and provided 

 with an anteriorly directed process. This part (fig. 334, M. Pt and Qu} 

 is then segmented from the lower region, and forms what may be 

 called the pterygo-quadrate cartilage, though not completely homo- 

 logous with the similarly named cartilage in Elasmobranehs ; 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 mandi- 

 bular 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 trabecuke, known as the ethmopalatine process. 



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

 tilage 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.Pt}; while the whole arch be- 

 comes firmly bcunl 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 (in.pg) 

 and a ventral pterygoid proper (py)- The quadrate region, articu- 

 lating with the Meckelian cartilage, becomes ossified as a distinct 

 quadrate (qu), while the dorsal region becomes also ossified as a meta- 

 pterygoid (mtpg}. 



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. 



