What Is a Mammal? 7 
mals assumes one of two rather distinct patterns. It may form 
a series of crescent-shaped ridges lengthwise in the jaw and 
with two or four crescents to each molar. Mammals which have 
developed such "selenodont" molars include the Phascolarctine 
Marsupials, the Colobid Monkeys, the majority of the Artio- 
dactyla (Cattle, Deer, Camels, etc.), some of the Philippine 
Complex-toothed Rats, some Flying Squirrels, and the Rabbits 
(a special type). The second pattern consists of a series of plate- 
like ridges one behind another, crossing the tooth from side to 
side. Those mammals that have adopted the plate-like or rigid 
pattern of crown surface include the Kangaroos, the Artio- 
dactyla (with the Rhinoceroses and Horses showing complex 
foldings), and to an extreme degree the living Elephants. 
Degeneration and obsolescence of the teeth in mammals, other 
than the Whales already mentioned, is initiated when diets of 
extremely soft and easily digested matter are habitually con- 
sumed. The South African Aard-wolf, a hysenid which eats 
chiefly grubs and termites, has small, weak teeth. The same is 
true of the Banded Civet, Chrotogale, which, according to 
Dr. Delacour, a scientist who has for many years lived in Indo- 
China, feeds on earthworms. The Armadillos, chiefly termite- 
eaters, and the Platypus, eater of water grubs, are approaching 
toothlessness. The Anteaters, Spiny Anteaters, and Pangolins, 
eaters of termites and other soft-bodied invertebrates, have no 
teeth at all. In view of this last, it may behoove our present 
generation of ice-cream lovers to watch their dental welfare 
closely. 
COMMON NAMES AND SCIENTIFIC NAMES 
When we speak of a Siberian Tiger, a Short-tailed Mongoose, 
a Pen-tailed Tree-shrew or a Black Rat we indicate with a rela- 
tively high degree of precision a certain kind of Tiger, Mon- 
goose, Tree-shrew, or Rat. But if we talk of a House Cat, a 
