Tetrabranchiata, and the absence of all evidence of the possession of
an ink-sac is in favour of this view. There can be little doubt that
they gave rise to the Dibranchiata.

About 2500 fossil species are included in the Nautiloidea, but only a
few species of the genus _Nautilus_ survive. Some of the fossil forms
are very large, the shell reaching a length of 2 metres, or 6 ft. 6
in. Of the Ammonoidea more than 5000 species have been described, and
some of the coiled forms are 70 cm., or nearly 2 ft. 6 in. in
diameter.

Associated with various forms of Ammonoids there have been found
peculiar horny or calcified plates, sometimes contained within the
body-chamber of the shell, sometimes wholly detached. The most typical
form of these structures has been named _aptychus._ It consists of two
bilaterally symmetrical halves, of somewhat semicircular shape, and
attached to one another by their straight inner margins, like a pair
of doors. In some cases the aptychus is thin and horny, but more often
it is thick and calcified, in which case the principal layer has a
peculiar cellular structure. The surface may be smooth or sculptured,
and one side is usually marked by concentric lines of growth. Another
type is similar, except that the two halves are united in the middle
line; bodies of this character are called _synaptychus_; they occur in
the body-chamber of species of _Scaphites_. Another form called
_anaptychus_ consists of a thin horny undivided plate which is
concentrically striated. This is associated with species of
_Ammonites_ and _Goniatites_.

Many theories have been proposed in explanation of these structures.
According to Sir Richard Owen, the aptychus is an operculum developed
in a part of the body corresponding to the hood of _Nautilus_. E. Ray
Lankester suggested that the double plate was borne on the surface of
the nidamental gland, with the form and sculpturing of which in
_Nautilus_ it closely agrees. On this view the aptychus would occur
only in females. The most recent view is that these structures could
not have been opercula because of their constant position inside the
body-chamber, and that they were not external secretions at all, but a
calcified internal cartilage situated at the base of the funnel.

_Classification of Tetrabranchiata._--Cephalopoda in which the mantle
is entirely enclosed by a multilocular siphunculated s