834 DIVISIONS OF THE CEPHALOPODA. 



eminently Paleozoic, only a few ' generic types surviving into the 

 Secondary period. On the other hand, the Ammonoidea are pre- 

 eminently Mesozoic in their range, being represented in the Palae- 

 ozoic deposits earlier than the Carboniferous only by such compara- 

 tively simple types as Clymenia and Goniatites. With but limited 

 and local exceptions, the Ammonoidea are not known to have sur- 

 vived into even the commencement of the Kainozoic period ; and 

 all the living Tetrabranchiates belong to the single genus Nautilus. 



Sub-Order I. Nautiloidea. 



In this division of the Tetrabranchiata the shell is straight or bent, 

 or variously coiled, the aperture being simple or contracted, and the 

 ventral side being commonly indicated by an emargination of the 

 apertural edge. The sutures are simple, rarely undulated or den- 

 ticulate ; and the septa are concave towards the shell-aperture. The 

 siphuncle is variable in position, and its cavity is often contracted by 

 internal deposits. The " septal necks " are usually short, though 

 sometimes long, and are generally directed backwards, the shell 

 being " retrosiphonate." The initial chamber is conical, and has a 

 cicatrix. 



This sub-order includes the genus Nautilus^ and a large number 

 of wholly extinct types. The general characters of the shell have 

 already been spoken of in dealing with the Tetrabranchiata as a 

 whole ; but there are a few special points which may be shortly 

 noticed here. 



The aperture of the body-chamber in the Nautiloids is variably 

 shaped. In some cases, as in Orthoceras, the aperture is simple, as 

 it is in the Pearly Nautilus ; but the lateral margins of the aperture 

 may be prolonged forwards, so that a sinus is produced on the con- 

 vex and concave sides of the shell. The 

 most marked and constant of these si- 

 nuses may be assumed to correspond with 

 the funnel, and therefore to represent the 

 ventral side of the animal ; and, as pre- 

 viously pointed out, this is in Nautilus 

 the convex side of the shell. While the 

 aperture in many Nautiloids is simple, 

 it becomes in other cases contracted by 

 Fig. 7 45.-Aperture of the shell the bending inwards of the lateral pro- 

 of Gomphoceras Bohemicum, re- l on gations of the mouth, so as to be 



duced in size. (After carrande.) ° ' 



converted into a dorsal and ventral open- 

 ing connected by an intervening passage. In some cases the 

 dorsal and ventral margins as well as the lateral ones are bent 

 inwards, in which case the aperture (fig. 745) assumes a T-shaped 



