ther, aperture simple, a horny anaptychus present.
_Ammonites_, Jurassic. _Arietites_, Jurassic. _Aegoceras_, Lias.

Fam. 8. _Harpoceratidae_. Shell discord and flattened, with a
carinated border, aperture provided with lateral projections, a
calcareous aptychus, formed of two pieces. _Harpoceras_, Jurassic.
_Oppelia_, Jurassic. _Lissoceras_, Jurassic and Cretaceous.

Fam. 9. _Amaltheidae_. Shell flattened, with a prominent carina
continued anteriorly into a rostrum. _Amaltheus_, Lias.
_Cardioceras_, Jurassic. _Schloenbachia_, Cretaceous.

Fam. 10. _Stephanoceratidae_. Shell not carinated, but with
radiating costae, which are often bifurcated, aperture often with
lateral projections which contract it, aptychus formed of two
pieces. _Stephanoceras, Morphoceras, Pensphinctes, Peltoceras_,
Jurassic. _Hoplites_, Cretaceous. _Acanthoceras_, Cretaceous.
_Cosmoceras_, Jurassic. Various more or less uncoiled forms are
related to this family, viz. _Scaphites, Crioceras_, Cretaceous.

ORDER 2. DIBRANCHIATA (= Holosiphona, Acetabulifera)

[Illustration: FIG. 15.--_Sepia officinalis_, L., about 1/2 natural
size, as seen when dead, the long prehensile arms being withdrawn from
the pouches at the side of the head, in which they are carried during
life when not actually in use. a. Neck; b, lateral fin of the
mantle-sac; c, the eight shorter arms of the fore-foot; d, the two long
prehensile arms; e, the eyes.]

_Characters_.--Cephalopoda in which the inflected margins of the
epipodia are fused so as to form a complete tubular siphon (fig. 24, i).
The circumoral lobes of the fore-foot carry suckers disposed upon them
in rows, _not_ tentacles (see figs. 15, 24). There is a single pair of
typical ctenidia (fig. 25) acting as gills (hence Dibranchiata), and a
single pair of renal organs, opening by apertures right and left of the
median anus (fig. 25, r) and by similar internal pores into the
pericardial chamber, which consequently does not open directly to the
surface as in _Nautilus_. The oviducts are sometimes paired right and
left (Octopoda, Oigopsida), sometimes that of one side only is developed
(Myopsida). The sperm-duct is always single except, according to W.
Keferstein, in _Eledone moschata._

A plate-like shell is developed in a closed sac formed by the mantle
(figs 20, 21), except in the Octopoda, which have none, and in _Spirula_
(fig. 17, D) and the extinct _B