72. MAMMALIAN DESCENT. [LECT. III. 



sides growing well forward into the Glaserian fissure, 

 also sends a large sickle of bone in front of, and within, 

 the bony tympanic ring ; thus, that bone has two 

 ancillary pieces, helping it to wall in the drum cavity. 



7. The innermost bone of the middle-ear chain is not 

 always stirrup-shaped, it is often a mere " colurnella," 

 or rod, with an oval dilated plate above, where it fits 

 into the oval window (fenestra ovalis), as in the 

 Monotremes and Ovipara. 



8. The angle of the lower jaw-bone is greatly 

 incurved, and often has on its upper and inner face a 

 hollow fossa, 



9. The hyoid bone is not simply U-shaped, but is 

 dilated into a wedge-shaped plate ; it has, however, 

 the usual cornua (horns or processes), that are indeed 

 the skeletal parts of the hyoid, or second arch, and 

 of the rudimentary third, or first gill arch. 



10. But the most interesting and instructive of all 

 the characters is one which at first sight would seem to 

 be a very small matter, but is indeed full of instruction, 

 namely, that the optic or visual nerve does not pass 

 from the brain to the eye through a special hole in that 

 part of the skull, but through a large chink in the 

 walls the common outlet for all the nerves of the 

 orbit. 



These are the ten good, useful, well-marked diag- 

 nostic characters, which I promised to show in the skull 

 of a Metatherian animal, or intermediate beast. 



