The Museum collection contains an incomplete skeleton of a very young
individual, Cat. No. 22875, which was received from Bering Island with
the skulls of _Z. grebnitzkii_, but does not belong to any one of them.
Whether it really represents that species is, therefore, uncertain, but
such is probably the case. The length of the vertebral column, consisting
of 45 vertebræ, without interspaces, is 9 feet 2 inches.
The vertebral formula is as follows: C. 7; Th. 10; L. 10; Ca. 18 (+1?) =
45 (+1?). This is the same as in the type of _semijunctus_ so far as the
cervicals, thoracics, and lumbars are concerned, and the probable total
is the same. In their general characters these vertebræ agree with those
of the skeletons already described, but they present a number of
differences as well. On account of immaturity the processes are even less
developed than in _semijunctus_. All the epiphyses are free, and in the
third to the seventh thoracic vertebræ the neural arch and spine are
separate from the centrum. The centra are very short in proportion to
their width.
Although the specimen is so young, the anterior foramen of the atlas is,
nevertheless, inclosed by bone, and though the line of separation between
the atlas and axis is visible on the sides, the fourth cervical is
anchylosed to the third at the top of the centrum. Although the neural
spines, metapophyses, and transverse processes of the thoracics are much
shorter than those of the young _semijunctus_, the epiphyses are as large
or even larger than in that specimen. The neural arches are also
noticeably thicker than in _semijunctus_, and the centra are rounded
inferiorly rather than carinated. The neural spines are much more nearly
erect than in the adult Barnegat and Newport skeletons, but, as mentioned
on page 41, this is probably a character of immaturity, and is shared by
_semijunctus_.
The differences as regards the form of the centra and neural arches die
away among the lumbars, and these vertebræ and the caudals are, with a
due allowance for greater immaturity, very similar to those of
_semijunctus_.
The seventh thoracic is like the sixth in form, and is without a
transverse process. It thus resembles the same vertebra in _semijunctus_.
The eighth, however, has an ill-defined facet on the side of the
metapophysis and a second facet a little above the upper border of the
centrum. The eighth pair of ribs has only a single terminal articular
facet.
The ninth thoracic has a short, thick transverse process, about in line
with the upper surface of the centrum.
The transverse process of the seventh caudal is perforated on the right
side by a foramen. The transverse processes are last traceable on the
ninth caudal, the neural spines on the tenth caudal, and the neural arch
on the eleventh caudal. Eight chevron bones are preserved, but probably
two more were present originally.
Ten pairs of ribs are present. The first is much broader in the proximal
half than in the distal half, but the distal end is slightly expanded.
The first seven pairs possess both head and tubercle, but the eighth,
ninth, and tenth have only a single terminal articular facet.
The sternum, which consists of five segments, is similar in form to that
of _semijunctus_. The two sides of each segment are united. The posterior
emargination of the third segment, and those of both ends of the fourth
and fifth segments are small. The scapula and humerus are like those of
_semijunctus_ in form. The remaining parts of both pectoral limbs are
lacking.
Without more material, and especially some skeletons of adults, it is
difficult to decide what importance should be assigned to the differences
observable in the cervical and thoracic vertebræ of this young Bering
Island specimen. The measurements of the skeleton are included in the
table on pages 47 and 48.