time above the alveolus as a hard tube until it either breaks across or falls out.
The teeth of the lower jaw, as already indicated, are much larger than those of the
upper jaw, and are not so curved as the latter, the crowns of which are set in at a
markedly obtuse angle to the root. The crowns of the teeth are worn flat from
without inwards and downwards. The greatest diameter is obtained at the middle
of the tooth, and thé fourth of the upper jaw is the largest ; the teeth behind it
decreasing in size to the last, which has a double curve and a conical, almost
unused crown. In the lower jaw the anterior tooth is quite as large as the teeth
behind it.
Subjoined are the measurements of the teeth of the specimen of Orcella
fluminalis under consideration :—
Upper jaw. Lowerjaw.
Inohes. Inohes.
1st tooth, right side, l e n g t h .......................................................................................................0'40 , 0‘70
»,' m breadth. . . . . . . . . . . 0 1 6 0‘24
4th „ „ length . ..................................................................... 0 62 0 68
„ „ breadth. . . . . . . . . . . 0'20 0 ‘20
Last tooth, „ l e n g t h ................................................................................. ......... 0 28 0 f36
,, „ breadth. . . . . . . . . . .0 * 0 8 0'16
Vertebral colimn.—There are in all 63 vertebrae, viz., C 7 : D 13 : L 16 and
Caudal 26. The atlas and axis are firmly soldered together (Plate XLIII, fig. 3), but
all the others, with the exception of the last cervical vertebra, are perfectly free.
The spinous process of these united vertebral elements is markedly bifurcate at its
extremity.
These amalgamated bones of the two species (Plate XLIII, figs. 3 and 8) present
certain characters by which they can be at once distinguished. In O.flummalis the
facets for the occipital are more widely divergent than in O. brevirostris, and
are separated at their inferior border by a much narrower interval. In the Irawady
dolphin the facets are almost oval, but in O. brevirostris they are much narrower
below than ahove. The height and breadth of the neural arch in the latter are proportionally
less than in the former, but the greatest breadth of the two bones is nearly
the same in an individual of O. brevirostris with a skull measuring l l -72 inches,
and in the specimen of O. fluminalis with its skull 12T6 inches in length. In the
latter species the breadth of the spinous process below its bifurcation is much greater
than in O. brevirostris, and the antero-posterior extent of the laminæ is much
greater in O. fl'uminalis. The remaining indications of the odontoid process are
also more strongly marked in the Irawady dolphin.
The lateral laminæ of the cervical veitebræ are most expanded in the 5th
segment, which is represented on Plate XLII, fig. 5, and which will suffice to givq a
general idea of the characters of these bones. The total length of the cervical region
in the fresh skeleton is 4-50 inches.
The first dorsal vertebra is figured on Plate XLII, fig. 6, reduced to one-half
natural size, and along with it the 7th dorsal and the 1st lumbar or that vertebra
opposite to which occurs the 14th or free floating rib. In structure this vertebra
belongs essentially to the lumbar region. The transition from the comparatively
short transverse processes of the dorsal vertebrae to the long processes of the lumbar
region is effected in the 13th dorsal in which the process suddenly lengthens, having
its external h a lf directed obliquely backwards. There is also a decided increase in
the length of the spinous process of the 13th dorsal vertebra, and it, along with the
transverse process, attains its maximum development on the 3rd lumbar, and from
that point both gradually decreasein size until the spinous process is lost on the
15th caudal and the transverse process on the 11th caudal. The neural canal
is perfect to the last process. Prom the 16th caudal the vertebrae rapidly diminish
in size, chiefly in their vertical depth, their transverse diameter more slowly
decreasing, until in the last segment only a round nodule remains 0T6 inch in breadth.
This little ossicle lies close below the skin in the notch between the caudal flippers.
The 5th and 6th caudal vertebrae are perforated, as usual by a canal passing through
the base of the transverse processes, but in the succeeding vertebrae the canal opens
on the under surface of each segment in the broad vascular furrow protected by the
chevron bones, but is also present in the 4th, 5th, and 6th caudals.
The chevron bones commence between the 12th and 13th lumbar vertebrae, that
.is, anterior to the position of the pelvic bones,—and have the general Cetacean
character. Their spines are well developed as far as the. 14th caudal, but beyond
that they become reduced to . a ridge and the bones themselves to nodules that can
he traced to near the end of the column. ■ '
jUls. There are in all thirteen true ribs (Plate XLII, I to xiii) on either side»
but a free floating rib occurs on both sides considerably removed from the vertebral
column and lying free in the lateral abdominal muscles. Pive are attached to.the
sternum by thoroughly ossified sternal ribs, the last being attached to the posterior
border of the sternum on either side of the notch. The 9th rib and those following
it are borne only on the transverse processes, the transition occurring rather suddenly
at the 8th vertebra.
Sternum.—This bone (Plate XLIII, fig. 10) differs considerably from that of
O. brevirostris (fig. 5), more especially in the dilated character of that portion to
which the first sternal rib is attached and in the full rounded sweep of that border
which in O. brevirostris is concave. I t is also distinguished from the latter by the
presence of a deep notch which separates the anterior wings. In the sternum of
O. brevirostris, however, there is a rounded hole occurring near the anterior margin
of the sternum in the mesial line which may mark what may have been the base of
a notch in the young skeleton afterwards closed in by the ossification of the
neighbouring margins. In 0. fluminalis the posterior end of the sternum is
sometimes divided in two by a deep wide notch, and in 0. brevirostris there
is occasionally an indication of such a division. The sternum of the latter is also
relatively broader than that of O. fluminalis.
Scapula.—The scapula of O. fluminalis differs from that of the marine form of
Orcella (Plate XLIII, figs. 2 and 6) in the greater length and expansion of the
post-scapular area, and in the more oval form (figs. 2 and 7) of the articular
surface for the head of the humerus.