Fig. 341.—Weasel, in summer; in Canada in winter it is all white but tip of tail.
Fig. 342.—Foot of Bear (Plantigrade).
Teeth and toes are the basis for subdividing the class mammalia into orders. Although the breathing, the circulation, and the internal organs and processes are similar in all mammals, the external organs vary greatly because of the varying environments of different species. The internal structure enables us to place animals together which are essentially alike; e.g. the whale and man are both mammals, since they resemble in breathing, circulation, and multiplication of young. The external organs guide us in separating the class into orders. The teeth vary according to the food eaten. The feet vary according to use in obtaining food or escaping from enemies. This will explain the difference in the length of legs of lion and horse, and of the forms of the teeth in cat and cow. Make a careful study of the teeth and the limbs as shown in the figures and in all specimens accessible. Write out the dental formulas as indicated at the top of page [194]. The numerals above the line show the number of upper teeth, those below the line show the number of lower teeth, in one half of the jaw. They are designated as follows: I, incisors; C, canine; M, molars. Multiplying by two gives the total number. Which skulls in the chart have the largest canines? Why? The smallest, or none at all? Why? Compare the molars of the cow, the hog, and the dog. Explain their differences. In which skulls are some of the molars lacking? Rudimentary? Why are the teeth that do not touch usually much smaller than those that do?
Fig. 343.—Polar Bear (Ursus maritimus).
| KEY, OR TABLE, FOR CLASSIFYING MAMMALS (class Mammalia) INTO ORDERS | ||||
|---|---|---|---|---|
| Orders | ||||
| A1 Imperfect Mammals, young hatched or prematurely born | ||||
| B1 Jaws a birdlike beak, egg-laying | Mon’otremes | |||
| B2 Jaws not beaklike, young carried in pouch | Marsu’pials | |||
| A2 Perfect Mammals, young not hatched, nor prematurely born | ||||
| B1 Digits with claws | C1 | Front part of both jaws lack teeth | Eden’tates | |
| C2 | Teeth with sharp points for piercing shells of insects | Insect’ivors | ||
| C3 | Canines very long, molars suited for tearing | Car’nivors | ||
| C4 | Canines lacking, incisors very large | Rodents | ||
| B2 Digits not distinct | C1 | Head large; carnivorous | Ceta’ceans | |
| C2 | Head small; herbivorous | Sire’neans | ||
| B3 Digits with nails or hoofs | C1 | Five toes, nose prolonged into a snout | Proboscid’eans | |
| C2 | Toes odd number, less than five | E’quines | Ungulates | |
| C3 | Toes even number, upper front teeth lacking, chew the cud | Ru’minants | ||
| C4 | Toes even number, upper front teeth present, not cud-chewers | Swine | ||
| C5 | All limbs having hands | Quad’rumans | ||
| C6 | Two limbs having hands | Bi’mans | ||
Exercise in Classification.—Copy the following list, and by reference to figures write the name of its order after each mammal:—
Rabbit (Fig. [345])