7& MAMMALIAN DESCENT. [Lect. IIL 



But in the lesser Tortoises the tympanic cavity is mad& 

 quite large by the hollowing out of both the quadrate 

 and the squamosal, whilst in Crocodiles and Birds 

 the whole hind skull, at any rate, is one system of 

 air-galleries, all communicating with the cavity of 

 the drum. 



In us, whatever kind of ear-drums our very first 

 parents may have possessed, there are no cells of this^ 

 kind except in the "mastoid process," the thick mass 

 below the labyrinth, which we feel as a lump behind 

 our ears. 



Character 4. — ^Whilst writing these notes, I have for 

 the first time found this fourth character in a mammal 

 above the Marsupials, namely, in an Insectivore from 

 Zanzibar (Rhynchocyon), a creature full of inconsist- 

 encies, but a treasure to the Darwinian. To him who 

 can wait, the whirligig of time brings its rewards as 

 well as its revenges. That mixed type (of which Dr. 

 Dobson was the kind donor) has come to me for the 

 establishment of my faith in development. 



Another equally valued friend. Professor Burt Wilder,, 

 of Cornell University, U.S., amongst other treasures, 

 sends me unborn embryos of the Virginian Opossum, and 

 now, after years of patient longing, I can compare the 

 development of this type of skull with that of the 

 Crocodile and the Bird. 



The process of cartilage that grows out on each side of 

 the second part of the skull-base, the hinder sphenoidal 



