8-20 



THE AMERICAN NA TL 'A\ I LIST. [Vol. XXXV II. 



A. External Modifications. 



1. Body more or less fusiform. — An obvious adaptation to 

 progression in such a dense medium as earth. In the common 

 mole (Condylura), for example, the body-diameter is greatest at 

 the shoulder, and diminishes gradually to a point at the nose. 

 In some fossorial forms, e. g. the wombat (Phascolomys) and 

 woodcluick (Arctomys), the body is very thick. 



2. Eyes impei feetly developed or obsolete.— Normally devel- 

 oped eyes are traditionally useless to an animal living in com- 

 plete darkness, and would be a continuous source of pain from 

 injury received in burrowing. The degree of degeneration 

 is no doubt partly dependent on the length of time which 

 has elapsed since the assumption of fossorial habits, and on the 

 relative completeness of withdrawal from the upper air. In 

 the pocket gophers (( ieomvida ) and Bathyergida: the eyes are 

 small ; in Spalax typhlus they are mere black specks among the 

 muscles (although retaining a relatively complete structure); in 

 the marsupial mole {Xotoryetes typhlops) they are imperfectly 

 developed and functionless ; in Talpa they are vestigial ; in the 

 Cape golden mole (Chrysochloris) the eyes are covered with 



3. External ears, small, tending to disappear. — External ears 

 impede burrowing especially as they are situated at the upper 

 and anterior part of the body where much friction would nat- 

 urally occur. Hence in the Geomyidae and in Lutra the exter- 

 nal ears are small, in the ratel (Mellivora) very minute and in 

 the Bathyergidae they have become reduced to a mere ring 

 of skin around the auditory aperture, while in Notoryctes, 

 Chrysochloris and Talpa they are absent. 



4. Limbs short and stout.— Since in a truly fossorial animal 



sion on the surface of the ground the limbs are short and stout 

 (Ornithorhynchus, Echidna, Talpa, etc.). This of course does 

 not hold good for the majority of semi-fossorial forms, as in their 

 life above ground they need speed either to get food or to 

 escape enemies. Many of these, however, as our common wood- 



