8 MAMMALIAN DESCENT. [Lect. III. 



In the Skate this element is a large triangular, 

 cartilage, on which swing both the first and second 

 arches of the face ; hence it has been termed the 

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

 arch, and the mandible 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." 



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

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

 where this plate is enlarged in relation to the larynx. 

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

 the Frog also it becomes a broad sheet, hke 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 key-stones of the series of 

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

 because 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. 

 I 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 

 the base of the skuU at this part ("orbi to-sphenoid") 



