336 THE CEPHALOPODA 
jections which contract it; aptychus formed of two pieces. Genera— 
Stephanoceras, Waagen ; Jurassic. Morphoceras, Douvillé ; Jurassic 
(Fig. 294).  Perisphinctes, Waagen ; Jurassic.  Peltoceras, Waagen ; 
Jurassic. Hoplites, Neumayr; Cretaceous. Acanthoceras, Neumayr ; 
Cretaceous. Cosmoceras, Waagen ; Jurassic. Various more or less com- 
pletely uncoiled forms are related to this family, viz. Scaphites, Parkinson ; 
Cretaceous. Crioceras, @Orbigny ; Cretaceous. 
ORDER 2. Dibranchia, Owen. 
In these Cephalopoda the external surface of the visceral mass 
is naked and is only protected by a more or less rudimentary 
shell, which is situated on the aboral surface and covered by the 
integuments of this region. The female Argonauta is the only 
member of the group that has a wholly external shell, but this is 
not adherent and is secreted by the dorsal arms. ‘The head of the 
Dibranchia bears eight acetabuliferous arms, and there is frequently 
a fifth pair of more or less retractile arms, situated between the 
third and fourth pair (Fig. 295, ée). The funnel is always a com- 
pletely closed tube (Figs. 287, 301, etc.). There are two branchiae 
and two kidneys, each of the latter having a pericardial orifice 
(Fig. 273, y). The cephalic cartilage is traversed by the oesophagus 
and encloses all the principal nervous centres. The ocular cavities 
are closed and the eyes have a crystalline lens (Fig. 283). Chromato- 
phores are present in the integument and an ink-sac is generally 
present. The Dibranchia include two sub-orders, the Decapoda and 
the Octopoda. 
Sus-OrpDER 1. Decapopa. 
In this sub-order, in addition to the eight pairs of normal arms, 
there is a more or less well developed ‘“‘tentacular” arm situated between 
the third and fourth normal arms, on each side of the head. These 
tentacular arms are more or less retractile within special pouches, and 
as a rule they only bear suckers at their free extremities. The suckers 
are pedunculated and provided with horny rings. The eight normal 
arms are shorter than the body. ‘There is generally a fairly well 
developed internal shell, and there are usually lateral fins of various 
width (Figs. 295, 296, etc., fi). The heart lies in a coelomic cavity. 
Nidamentary glands are usually present. 
The Decapoda comprise two tribes, the Oigopsida and the Myopsida. 
TRIBE 1. OIGOPSIDA. 
The members of this tribe are characterised by the presence of a 
wide orifice, occupying the optic axis, in the external false cornea of the 
eye. As a rule two oviducts are present. In the fossil genera the shell 
has a multilocular phragmocone with a siphuncle ; the initial chamber of 
this shell is globular and larger than the second chamber. The most 
ancient forms are characterised by the small size of the rostrum, the 
