NAUTILOIDEA. 



835 



or key-hole-like character. iVccording to the views of Barrande, 

 the broad transverse part of the aperture represents in such cases 

 the position of the head, while the rounded extremity of the verti- 

 cal portion of the aperture corresponds with the funnel. It is 

 clear, however, that with an aperture so contracted, the animal 

 must have been incapable of protruding the head, as the living 

 Pearly Nautilus can. 



The dorsal and ventral aspects of the animal have no constant 

 relation to the concave and convex sides of the shell among the 

 Nautiloidea. In the Pearly Nautilus the convex side is the ventral 

 one, but even within the limits of the same genus it is usual to meet 

 with forms having the convex side 

 ventral associated with others in s 



which the concave side is ventral. 



The position of the siphn?ide is 

 by no means constant among the 

 Nautiloids, and bears no fixed rela- 

 tion to the aspects of the body of 

 the animal, though it is commonly 

 placed towards the ventral side. 

 The siphuncle is upon the whole, 

 however, most generally situated 

 centrally or subcentrally, and when 

 removed towards the margin it is 

 usually external in the curved 

 shells, though it may be internal. 

 As regards its structure the siph- 

 uncle of the Nautiloids is, to begin 

 with, a simple membranous tube, 

 but it may become complex in 

 process of growth. In many cases 

 the cavity of the siphuncle be- 

 comes contracted by the formation 

 in its interior of annular deposits 

 of calcareous tissue (fig. 746, or), 

 which were termed by Barrande 



" nhxsl-mrl-inn rinoV • anrl rhpsp mnv Fi §- 746-— Vertical section of Orthoceras 



ODstruction-rmgs , ana tnese ma> angu ^ tumi Silurian, showing the siphuncle 



be SO extensively developed as tO 0) contracted by the development of " ob- 

 , i -, i i struction-rings " {or) opposite the septa. 



completely obstruct the tube. (After Foord.) 



The " septal necks " of the Nau- 

 tiloids are almost always directed backwards, the shell thus being 

 "retrosiphonate" (fig. 743). In the two genera Nothoceras and 

 Bathmoceras alone are the septal necks turned forwards, so that 

 the shell is " prosiphonate." In many forms, as in the existing 

 Pearly Nautilus, the septal necks are short ; but in other cases 



