210 ME. W. K. PAEKER ON THE STEUCTURE AND DEVELOPMENT 



The pterygo-mandibular arch (PI. XXXVII. fig. 2, and PI. XXXVIII. figs. 1 & 2, 

 q.pg, q, mn, mk) is curiously swung from the outside of the basis cranii by two short 

 ropes of fibrous tissue : the hindermost of these is the true apex of the arch, the meta- 

 pterygoid, or " pedicle" (Z, sp.c); and the foremost is the palato-trabecular conjugation 

 {l).tr). This latter is attached to a process on the pterygoid, too short to reach the 

 trabecula; and the metapterygoid band in front of the spiracle {cl. I) is attached 

 above to the skull, close in front of the hyomandibular {hm), and behind and below 

 the fifth nerve (5). Where this band is attached to the quadrate region {q), the 

 cartilage is bevelled down, only sufficient substance being left to form the condyle for 

 the mandible. 



There is, however, but little difference in size between the upper and lower 

 jaw-plates ; for the upper broadens as it arches upwards to the dentigerous part, and 

 then it gently curves downwards to its pterygoid or distal end, which is strongly tied 

 by a ligament to its fellow of the opposite side. This bar is bowed out at its upper 

 margin, as is the mandible at its lower edge ; the scooped surfiice thus formed gives 

 attachment to the oral muscles. This primary mandible has a very similar form to 

 the secondary mandible of a mammal, save in not possessing a coronoid process. Its 

 narrowing dentigerous part is equal in size and extent to that of the upper bar ; the 

 right and left halves are similarly bound together by hgamentous fibres. 



The hyoid arch is as simple as the mandibulo-pterygoid, being composed of two pieces 

 only ; the whole arch cleaves close to the one in front, and is locked within it below. 

 This is a foreshadowing of what takes place in more metamorphosed types, as is also the 

 suspension of both the free crura from the pier of the hyoid, which now can fairly be 

 called, from its function, the hyomandibular {hm). The metapterygoid ligament is 

 attached below equally to the two contiguous condyles — namely, that of the quadrate 

 and of the hyomandibular ; thus the weight of the large mandible is transferred largely 

 to the hyoid pier. The hyomandibular has a joint-cavity above and below it; and its 

 articulation with the free cornu below is like that between the human finger-joints. 



The thick, ribbed, outturned edge of the mandible is attached at its rounded angle, 

 near the joint, by a strong " mandibulo-hyal " ligament (m.h.I), as in the Urodeles; in 

 Osseous Fishes the angular ligament becomes bony above, as the " interopercular." 

 The stout phalangiform ceratohyal (c.hy) by this ligament is kept close to and within 

 the mandible ; it is bilobate distally ; and the anterior lobe articulates with the angle of 

 the elegant heart-shaped basihyal (PI. XXXVIII. fig. 1, c.hy, h.hy) ; there is the merest 

 rudiment of a joint-cavity at this part, but, as in the arches behind, a profusion of 

 fibrous tissue. 



As the hyoid arch is functionally branchial, it carries branchial rays {hr.r), that give 

 slrenoth to the pouches. There are three of these on the hyomandibular, the upper 

 of which is trifurcate, the middle bifurcate, and the lower subdivided into five or six 

 long leaflets. 



