MOLLUSCA. CEPHALOPODA 



263 



sutures ; usually they are either straight or only slightly 

 undulating. 



The shell which covered the embryo in the Cephalopoda 

 is known as the protoconch 1 ; in 

 the Nautiloidea this probably con- 

 sisted of non-calcified material, 

 and is not definitely known to 

 be preserved in fossil specimens ; 

 but some authors have found 

 calcareous protoconchs in straight 

 conical shells which they be- 

 lieve to belong to the genus 

 Orthoceras. If the shell of a 

 Nautilus be sliced in two, there 

 will be seen at the centre a space 

 (fig. 104, a) which was probably 

 occupied by the protoconch. At 

 the apex of the first chamber is an 

 opening which is usually slit-like and surrounded by 

 either a rim or a depression ; this opening probably 

 served to connect the protoconch with the first chamber. 

 The siphuncle (c) commences in the first chamber as a 

 closed tube. 



Fig. 104. Median section 

 of the central part of the 

 shell of Nautilus, a, cen- 

 tral space ; b, septum ; 

 c, siphuncle ; d, septal 

 neck. Enlarged. 



Orthoceras. Shell straight or occasionally slightly curved, 

 elongate-conical ; transverse section usually circular. Septa con- 

 cave ; body-chamber large ; aperture not contracted or produced 

 into lobes. Siphuncle cylindrical, without internal calcareous 

 deposits ; usually central, but sometimes sub-central or ex-central. 

 Ornamentation variable. Orthoceras is divided into several sub- 

 genera. Tremadoc Beds to Trias. Ex. 0. annulatum, Wenlock 

 Limestone ; 0. goldfussianum, Carboniferous Limestone. 



1 This corresponds to the protegulum of the Brachiopods and to the 

 prodissoconch of the Lamellibranchs. 



