TETRABRANCHIATA. 



829 



" endogastric " types the concave side of the shell is the ventral 

 side, the dorsal side being convex. In a great many fossil forms, 

 however, there is no clear evidence as to the position of the animal 

 in relation to its shell ; and in such cases, as in the Ammonites, it 

 is advisable to employ the terms " external " and " internal " for the 

 convex and concave sides of the shell, since we do not know which 

 side is "dorsal" or "ventral." 



The shell of the Tetrabranchiates may be regarded as essentially 

 a more or less elongated cone, the pointed end of which is parti- 



Fig. 740. — Fragment of Actinoceras 

 crebriseptu7ii — Cincinnati Group, North 

 America, of the natural size. The lower 

 figure is a section showing the air- 

 chambers, and the form and position 

 of the siphuncle. (After Billings.) 



Fig. 741. — Restoration of Orthoceras, 

 the shell being supposed to be divided 

 vertically, and only its upper part being 

 shown. «, Arms ; f, Muscular tube 

 ("funnel") by which water is expelled 

 from the mantle-chamber ; c, Air-cham- 

 bers; s, Siphuncle. 



tioned off into air-chambers, while the body-chamber is situated at 

 the wide end (figs. 740 and 741). In some cases, as in Orthoceras 

 and Baculites, the shell retains permanently its primitive form as a 

 straight cone. In other cases the originally straight cone may be 

 slightly bent ( Cyrtoceras) ; twisted into a hook (Hamites) ; doubled 

 on itself {Ptychoceras) ; or coiled into an open spiral, tlie turns of 

 which may lie in one plane (Gyroceras) or may pass obliquely round 

 an imaginary axis (Helicoceras). Very generally the volutions of the 



