nd narrow, with numerous
teeth in both; the symphysis of the lower one exceeding half its
length. Externally the head is divided from the body by a slightly
constricted neck. Pectoral limbs broad and truncated. Dorsal fin small
or obsolete. In habits these dolphins are fluviatile or estuarine. In
the Indian susu, or Ganges dolphin (_Platanista gangetica_), the teeth
number about 30/30 on each side, are set near together, are rather
large, cylindrical, and sharp-pointed in the young, but in old animals
acquire a large laterally compressed base, which in the posterior part
of the series becomes irregularly divided into roots. As the conical
enamel-covered crown wears away, the teeth of the young and old
animals have a totally different appearance. The beak and
tooth-bearing portion of the lower jaw are so narrow that the teeth of
the two sides are almost in contact. Maxillae supporting large,
incurved, compressed bony crests, which overarch the nostrils and base
of the rostrum, and almost meet in the middle line above. Orbits very
small and eyes rudimentary, without crystalline lens. Blow-hole
longitudinal, linear. Vertebrae: C 7, D 11, L 8, Ca 25; total 51. A
small caecum. No pelvic bones. Dorsal fin represented by a low ridge.

[Illustration: FIG. 7.--River Plate Dolphin (_Stenodelphis
blainvillei_).]

The second genus is represented by _Inia geoffroyi_, of the Amazon, in
which the teeth vary from 26 to 33 pairs in each jaw; those at the
posterior part with a distinct tubercle at the inner side of the base
of the crown. Vertebrae: C 7, D 13, L 3, Ca 18; total 41. Transverse
processes of lumbar vertebrae very broad. Sternum short and broad, and
consisting of a single segment only. Dorsal fin a mere ridge. The long
cylindrical rostrum externally furnished with scattered, stout and
crisp hairs. The third type is _Stenodelphis blainvillei_, the River
Plate dolphin, a small brown species (fig. 7), with from 50 to 60
pairs of teeth in each jaw, furnished with a cingulum at the base of
the crown. Jaws very long and slender. Vertebrae: C 7, D 10, L 5, Ca
19; total 41. Transverse processes of the lumbar vertebrae extremely
broad. Sternum elongated, composed of two segments, with four sternal
ribs attached. Dorsal fin rather small, triangular, pointed. Blow-hole
transverse. In several respects this species connects the two
preceding ones with the _Delphinida