8 3 2 



DIVISIONS OF THE CEPHALOPODA. 



Ammonoid Tetrabranchiates have the septal necks continued 

 forwards from the producing septa, and are therefore said to be 

 "prosiphonate." In many of the extinct types of the Nautiloidea 

 the internal structure of the siphuncle is highly remarkable, the 

 cavity of the tube often becoming contracted by organic deposits of 

 secondary origin ; but the structures in question will be more fully 

 considered later on. Lastly, important distinctions are drawn in 

 the Tetrabranchiates from the position of the siphuncle in relation 

 to the shell. In the Amtnonoidea the siphuncle is always marginal, 

 and is mostly placed on the external side of the shell. In the Nau- 

 tiloidea, on the other hand, the siphuncle may pierce the septa 

 centrally (as in the Pearly Nautilus) ; or it may be subcentral ; or in 

 other cases it may be marginal, being then sometimes external, 

 sometimes internal in position (fig. 742). 



The development of the shell of the Pearly Nautilus is, unfor- 

 tunately, unknown ; but it has been shown that there are important 

 distinctions in the form and structure of the initial chamber of the 



Fig. 744. — Development of Tetrabranchiate Cephalopods. a, The inner end of the shell of 

 Nautilus pompiliiis, enlarged, showing the initial chamber and the cicatrix ; b, Cyrtoceras 

 pr&posteriiin—'S\\\±T\<vc\, showing the commencement of the shell; and (c) the initial chamber 

 viewed from below, showing the cicatrix ; D, Inner portion of the shell of Gonitztites bicanalic- 

 ulatus — Devonian, showing the inflated protoconch ; E, First turn of the spire of Goniatites sub- 

 lamellosus — Devonian ; f, Crioceras Studeri — Cretaceous, enlarged, showing the protoconch ; 

 G, Protoconch and first turn of the spire of Ammonites quadrisulcatus — Cretaceous, enlarged. 

 (After Barrande.) 



shell in the two groups of the Nautiloidea and Ammonoidea respec- 

 tively. In the Ammonoids the initial chamber of the shell (fig. 

 744, d — g) constitutes an inflated, spheroidal, oval, or pyriform sac, 

 the so-called " protoconch " (or " ovisac "), which corresponds with 

 the " nucleus " of the shell of the Gastropods, and is separated by a 

 constriction from the first air-chamber. The siphuncle commences 

 as a closed and dilated tube, which deeply indents the front wall of 



