THE SKULL. 



475 



together, and the cartilages of the two sides composing tliem meet 

 each other distally. 



At the articulation of the Meckelian cartilage with the quadrate 

 part of the pterygo-quadrate is situated a ligament (M.Pt), which takes 

 the place of the metapterygoid process of the previous stage, and passes 

 up on the anterior side of the spiracle, to be attached to the cranium 

 in the front part of the auditory region. This ligament, which is 

 supplemented by a second ligament the ethmopalatine ligament, 

 passing from the pterygo-quadrate bar to the antorbital region of the 

 skull, is not the most important support of the jaw. The main sup- 

 port is, on the contrary, given by the hyoid arch; the hyomandibular 

 segment of which [H.M) as well as the adjoining segment (cerato- 

 hyoid G.Hy) are firmly attached by ligament to the mandibular arch. 

 The hyomandibular is articulated with the cranium beneath the 

 pterotic ridge {Pt. 0). 



In the type just described, the hyoid and mandibidar arches 

 undergo less modification than in almost any other case. The 

 hyoid arch has altered its form, but retains its respiratory func- 

 tion. It has however acquired the secondary function of supporting 

 the mandibular arch. The mandibular arch is divided into two 

 elements, which form respectively the upper and lower jaws. It is 

 not directly articulated with the skull, and its mode of support by the 

 hyoid arch has been called by Huxley (No. 445) hyostylic. 



The development of the hyoid and mandibular arches in the 

 Skate is characterised by a few important features (fig. 333). The 

 anterior element of the hyoid 



arch, which forms the hyoman- C.a j^« M.p ^^ Sp 



dibular(//.ilf), becomes entirely 

 separate from the posterior part 

 of the arch, and only serves to 

 support the jaws. The poste- 

 rior part of the arch {Hij) 

 carries on tlie respiratory func- 

 tions of the hyoid, and is closely 

 connected with the first bran- 

 chial arch. The upper or me- 

 tapterygoid element of the 

 mandibular arch {M.Pt) has a 

 considerable development, and, 

 becoming separated from the 

 remainder of the arch, forms a 

 mass of cartilage with one or 

 two branchial rays, in the front 

 wall of the spiracle, and con- 

 stitutes a section of the mandibular arch still retaining traces 

 of its primitive function in supporting the wall of a branchial 

 pouch. 



Although the development of other Elasmobranch types is not 



Fig. 333. Head of embryo Skate, 1^ 

 IN. LONG. (From Parker.) 



Tr. trabecula ; Fl.Pt. pterygo-quadrate 

 bar; Mn. mandibular bar; M.Pt. meta- 

 pterygoid cartilage; H.M. hyomandibu- 

 lar; Hy. remainder of hyoid arch; Br.l. 

 first branchial arch ; Sp. mandibulo-hyoid 

 cleft or spiracle ; Fn. pineal gland ; Au. au- 

 ditory vesicle; C. 1, C.2, and (7.3. vesicles of 

 the brain. 



