336 THE CEPHALOPODA 



jections which contract it ; aptychus formed of two pieces. Genera 

 Stephanoceras, Waagen ; Jurassic. Morplwceras, Douville ; Jurassic 

 (Fig. 294). Perispkinctes, 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, d'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, te). 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. 



SUB-ORDER 1. DECAPODA. 



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., ft). 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 



