42 MAMMALIAN DESCENT. [Lect. II. 



it a third, the coronoid. These can also be traced in 

 the jaw of a young Ornithorhynchus, but they are only 

 semi-distinct. 



Then the fore part — about two-thirds of the primary 

 cartilaginous jaw — called " Meckel's " cartUage, is ossified 

 whilst the pinching off is taking place. 



Here then let there be an end to aU talk about the 

 simplicity of the lower jaw in the Mammalia. After 

 Nature has removed the hinder part, which in reptUes 

 and birds is itself composed of three external and one 

 internal bony centre, there stUl remain in the inferior 

 maxUla, or lower jaw, of a mammal, the following 

 elements, namely, (l) the dentary bone, with rudi- 

 ments of the splenial and coronoid ; (2) a large super- 

 ficial cartUage, or " inferior labial ;" and (3) the distal 

 two-thirds of the primary lower jaw, or Meckel's carti- 

 lage. 



Now, as to its suspension to the skull, this of 

 •course is m front of the old swinging point of the non- 

 segmented jaw of the ovipara. In them the quadrate, 

 or huge prototype of the "incus," or anvil hone, is 

 attached to the skull over the tympanic cavity, and 

 that^is also the place where the incus is always 

 found in the mammal. 



In them also (the ovipara) the squamous element, or 

 temporal scale-bone, has no cartilage on its under sur- 

 face. In the bird, for instance, the " zygomatic process " 

 is a mere snag for muscular attachment, for it has no 



