134 THE DOLPHIN. 
six to ten feet in length, seven feet being the usual average. Its colour is 
vlack upon the back, and silvery white upon the abdomen; while the flanks 
are greyish white. ‘he beautiful colours which have been said to play about 
the body of a dying Dolphin are 
not entirely mythical, but belong 
rightly to one of the fishes, the 
coryphene, or dorado, which is 
popularly called the Dolphin by 
sailors, 
The eyes of the Dolphin are 
small, and are supplied with eye-_ 
lids ; the pupil of the eye is heart- 
shaped. The ears have but a very 
DOLPHIN.—(Lirdp.inus Delphis.) minute external aperture, barely 
admitting an ordinary pin. 
It is a lively and playful animal, and being remarkably active in its native 
element, is fond of gambolling among the waves, and engaging in various 
sports with its companions. Being of a very gregarious nature, it is seldom 
seen alone, but prefers to associate in little flocks or herds, and is in the habit 
of accompanying ships for considerable distances, hovering about the vessel 
and executing various strange manceuvres. 
The Dolphin only produces a single young one at a time, and nurses her 
offspring with exceeding tenderness and assiduity. 
The common Dolphin is found in the European seas, and in the Atlantic 
and the Mediterranean, and may possibly have a still wider range. 
RODENTS. 
THE RODENTS, or gnawing animals, derive their name from the peculiar 
structure of their teeth, which are specially fitted for gnawing their way 
through hard substances. The jaws of the Rodents are heavily made, and 
very large in proportion to the head, their size being not only needful for the 
synport of the gnawing teeth, but for their continual development. There 
arc no canines, but a wide gap exists between the incisors and the molars, 
which are nearly flat on their surfaces, and are well suited for grinding the 
soft substances on which these animals feed. 
The structure of the chisel-edged incisor teeth is very wonderful, and may 
be easily understood by inspecting the teeth of a rat, mouse, hare, or rabbit. 
As their teeth are continually worn out by the severe friction which they 
undergo continually, there must needs be some provision for renewing their 
substance, or the creature would soon die of starvation. In order to obviate 
this calamity, the base of the incisor teeth pass deeply into the jaw-bone, 
where they are continually nourished by a kind of pulpy substance from which 
the tooth is formed, and which adds fresh material in proportion to the daily 
waste. 
Something more is needed for the well-being of the animal than the mere 
growth of its teeth; for unless their chisel-like edges were continually kept 
sharp, they would be of little use for cutting their way through the hard 
substances which the Rodents are in the habit of gnawing. This result is 
attained as follows :— 
The enamel which covers the front face of the incisor teeth is much harder 
than that which is laid upon the remaining surfaces, while the dentine which 
makes up the solid mass of each tooth is also harder in front than behind. 
It is evident that when these teeth are employed in their usual task, the softer 
