CEPHALOPODA. 69 



Fam. Nautilidae. The septa are simply bent and concave towards the 

 anterior chambers. Line of suture simple, with a few large wavy curves or a 

 lateral lobe. Siphon usually central ; shell orifice simple. Ortlioceras, shell 

 straight. 0. regularise. Schl., calcareous strata of the North German plain. 

 Nautilus, shell coiled. N. pompilius L. , Indian Ocean. 



Fam. Ammonitidae. The septa much folded at the sides, always with lobe 

 on the outer side, in the middle usually convex forwards. Siphon on the 

 outer side. Contains only fossil forms. Goniatites retrorsus v. Buch., Cera- 

 tites nodosus Bosc., Ammonites capricovnus v. Schl. 



Order 2. DIBRANCHIATA.* 



Cephalopoda with two gills in the, mantle cavity, eight arms bearing 

 suckers or hooks, complete funnel and ink-sac. 



The Dibranchiata possess round the mouth eight arms provided 



FIG. 543. Argonauta argo (female), swimming. 



with suckers or hooks ; in the Decapoda there are, in addition, two 

 long tentacles placed between the ventral arms and the mouth. The 

 cephalic cartilage constitutes a completely closed ring surrounding 

 the central parts of the nervous system ; its slightly arched lateral 

 parts serve for the support of the sessile eyes. There are only two 

 gills in the mantle cavity and the same number of branchial vessels 

 and kidneys. The funnel is closed. An ink-sac is usually present. 

 The shell is in many forms completely absent ; in others it is reduced 

 to a horny or calcareous dorsal lamella. A spirally-coiled shell is 



* Chief works : Ferussac et d'Orbigny ?.., also Verany I.e. 



