PORPOISES AND DOLPHINS. 4I 
More remarkable, however, is the occurrence of another nearly-allied form 
(Zniopsis) in the Tertiary of the Caucasus, which serves to confirm the view that 
the Platanistidw were a very ancient and widely-distributed family. Numerous 
other extinct dolphins, some of which approach those of the next family, have been 
described from North America and Europe, and referred to the present group. 
THE PORPOISES AND DOLPHINS. 
Family DELPHINID. 
The whole of the remaining members of the existing Cetacea, including those 
commonly known as narwhals, porpoises, grampuses, and dolphins, are referred to 
a single family, which contains a far greater number of species than any of the 
others. All the members of this family are of small or medium size, and, with the 
exception of the narwhal and Risso’s dolphin, are furnished with a numerous series 
of teeth in both the upper and lower jaws, the jaws themselves being either 

SKELETON OF DOLPHIN. 
elongated or short. They are distinguished from the preceding family by the 
union between the two branches of the lower jaw being always considerably less 
than half the entire length of the jaw itself, and likewise by the manner in which 
the ribs are articulated to the backbone, as well as by certain features in the base 
of the skull. The vertebree of the neck are generally characterised by the first 
two, three, or four being united together. The blowhole is always in the form of a 
transversely-placed crescent, with its two horns directed forwards. 
The majority of the dolphins (as it is convenient to term 
collectively the whole of the members of the family) are of marine 
habits, but many of them frequent estuaries and ascend tidal rivers, while a few 
are more or less exclusively fresh-water. They have been divided into a considerable 
number of genera, but many of these are very closely allied, and mainly distinguished 
by differences in the skeleton. 
Habits. 
THE NARWHAL. 
Genus Monodon. 
The strange-looking animal commonly known as the narwhal (Monodon 
monoceros) differs from all other members of the family by the enormous spirally- 
twisted tusk projecting from one side of the upper jaw of the male. This tusk is 
nearly always that of the left side, its fellow on the opposite side being only a few 
inches in length, and lying entirely concealed within the bone of the jaw, while in 
