THE SKULL. 



577 



Au 



arch. The hyomandibular is articulated with the cranium 

 beneath the pterotic ridge (P/.O). 



In the type just described, the hyoid and mandibular 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 sup- 

 porting 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 hyo- 

 mandibular (H.M}, becomes 

 entirely separate from the 

 posterior part of the arch, and 

 only serves to support the 

 jaws. The posterior part of 

 the arch (Hy) carries on the 

 respiratory functions of the 

 hyoid, and is closely connec- 

 ted with the first branchial 

 arch. The upper or meta- 

 pterygoid element of the man- 

 dibular arch (Rl.Pf] 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 known, it is necessary to call attention to the mode of 

 support of the mandibular arch in certain forms, notably Noti- 

 danus, Hexanchus and Cestracion, where the pterygo-quadrate 

 region of the mandibular arch is directly articulated to the 



B. III. 37 



FIG. 333. HEAD OF EMBRYO SKATE, ij 

 IN. LONG. (From Parker.) 



Tr. trabecula ; Pl.Pt. pterygo-quadrate 

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

 pterygoid cartilage ; H.M. hyomandibu- 

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

 first branchial arch ; Sp. mandibulo-hyoid 

 cleft or spiracle ; Pn. pineal gland ; An. au 

 ditory vesicle ; C. i, C. -2, and C. 3. vesicles 

 of the brain. 



