80 MAMMALIAN DESCENT. [Lect. III. 



Ill the Skate this element is a hirgc triangular 

 cartilage, on which swing l)otli the first and second 

 arches of the face ; hence it has heen termed the 

 " hyo-mandihular," for it carries the hyoid or tongue 

 arch, and the mandiljle also. 



Character 8. — The incurved form of the lower jaw at 

 its hind angle is not confined to the Marsupials ; it is 

 also seen to a less degree in various kinds of lower 

 Eutheria. In the types beneath, it simply represents 

 the large development of the dentary bone ; in many of 

 these it forms a. " trough," and grows freely on the inner 

 side of " Meckel's cartilage." 



Character 9. — The basi-hyal plate, or skeleton of 

 the back of the tongue, is seen in the higher mammals, 

 where this })late is enlarged in relation to the larynx. 

 In Turtles and Crocodiles this plate is flat and wide ; in 

 the Frog also it becomes ta lu-oad sheet, like an apron, 

 the narrowed bands of the arch being the strings. But 

 in fishes there is a chain of segments at this part, the 

 basi-hyobranchial series of hv/-.stones of the series of 

 gill-arches. It is thus short in the air-breathing types, 

 l)ecause of the suppression, in them, of the gill-arches. 



Character 10. — This is a "small wire," but it will 

 bear to have a large amount of induction hung upon it. 

 T know of no case in the existing Vertebrata below 

 mammals, where the optic or visual nerve is protected 

 by a special bar behind it from the plate that forms 

 1 he base of the skull at this part ('' orbito-sphenoid") 



