210 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
The pterygo-mandibular arch (Pl. XX XVII. fig. 2, and Pl, XXXVIII. figs. 1 & 2, 
q-P9, 7, 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” (/, sp.c); and the foremost is the palato-trabecular conjugation 
(p.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 (¢/. 1) is attached 
above to the skull, close in front of the hyomandibular (4m), and behind and below 
the fifth nerve (5). Where this band is attached to the quadrate region (gq), 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 surface 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 ligamentous 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 (dm). 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./), as in the Urodeles; in 
Osseous Fishes the angular ligament becomes bony above, as the “ interopercular.” 
The stout phalangiform ceratohyal (c./y) 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 (P]. XX XVIII. fig. 1, c.hy, 6.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 (47.r), that give 
strength 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. 
