MOLLUSCA — CEPHALOPODA. 



149 



this mode of growth (Fig. 80 h). The shrinkage appears to ^^]^^ 

 have been greater from the first, and thus the septa stretch Table-cas 

 across the conical shell, dividing it into a series of chambers, 1. 

 and leaving only a narrow neck-tube in which there are no 

 sheaths. The part of the shell-cavity in front of the last- 

 formed septum is called the body-chamber, and in it was the 

 main mass of the animal. From the visceral hump, how- 

 ever, proceeded the fleshy siphuncle, passing through each 

 septum to the apex of the shell. The line along which the 

 septum is attached to the shell-wall is called a suture, and in 

 these simple forms passes regularly all round the shell. 



There are numbers of straight shells of this simple type, 

 but those that are the most completely known may be divided 

 into two groups by the presence or absence of a small, more 

 or less globular, initial chamber. This, which is generally 

 separated from the next chamber by a slight constriction, is 

 called the protoconch (first shell) and believed to be the shell 

 of the embryonic cephalopod. It is well seen in some specimens 

 of Bactrites (Fig. 81 k). Often this protoconch seems to have 

 been lost in the adult, and in its place is seen only a scar or 

 cicatrix denoting its former presence (Fig 81, c, e, m). 



At an early period in the history of the cephalopod race 

 the shell began to curve, and this curvature increased until 

 the shell was coiled on itself. Such a coiled shell was far 

 more manageable than the long shell of an Orthoceras, and 

 was less liable to damage. And so it is found that the long 

 straight shells gradually die out and give place to coiled 

 shells. Now, just as there were two types of straight shells, 

 so were there two of coiled shells : one with a protoconch, as 

 may be seen in models of early goniatites (Fig. 81 n) \ the 

 other without a protoconch, as shown by the model of 

 Nautilus (Fig. 81 a-c). Further examination of the shells 

 of these two types reveals other differences. The early 

 coiled shells with a protoconch are long, narrow, smooth, 

 with septa usually far apart, and with a long deep body- 

 chamber (Fig. 81 n). Those without a protoconch are short, 

 broad, often with a longitudinal ornament, with septa 

 relatively close together, and with a shallow body-chamber 

 (Fig. 81 a). In later forms of these two types other 

 differences appear, such as will be realised by comparing 

 an ammonite (which is one of the former series) with a 

 Nautilus (Fig. 82). Generally speaking the siphuncle of an 

 ammonite is close to the outside of the coiled shell ; the 

 edges of the septa are folded, so that the sutures are compli- 



