MOLLUSCA — CEPHALOPODA, 



173 



then returns for a longer distance, and ends straight after Gallery 

 yet another bend (Fig. 96 e) ; and Baculites, in which, after a Table-case! 

 small initial coil, the shell continues straight to its aperture 14 & 15. 

 right up to death (Fig. 96 g). By study of tlie ornament Wall-case 

 and suture, it has been found that these forms represent ^' 

 stages of degeneration common to more than one race, and 

 therefore do not constitute true genera. Thus many of the 

 ISTeocomian shells in the Crioceran stage are ribbed in the 

 irregular manner characteristic of Lytoceras ; see, for example, 

 C. mllersianum in the Table-case. Those named Pictetia 

 Astieriana also show the peculiar Lytoceran suture. In 

 many of those from the Speeton Clay, on the other hand, 

 e.g. C. qicadrahcm, the ribbing is very irregular, and appears 

 to have been derived from such a form as Ferisphinctes or 

 Holcosteplmmis. The same appears to be the case with 

 the large Aptian Cnocems Boiverhanki and its derivative 

 Macroscaphites grandis, the earlier whorls of which resemble 

 those of the Portlandian Holcostephanus gigas, while the later 

 portion presents a remarkable exaggeration of certain ribs. 

 In these massive shells may be seen a curious retention by 

 the uncoiled portion of a character originally due to close 

 coiling. The cone in the earlier Ammonoidea is circular or 

 elliptical in section, but as they become coiled the inner side 

 of the outer whorls is impressed or excavate, so as to fit 

 closely over the inner whorls (Fig. 95 &) ; the closer the coil, 

 the greater is the depth of the impressed zone. This 

 impressed zone is clearly seen on the later whorls of many 

 of these uncoiled forms and is clear evidence of their 

 descent from more closely coiled ancestors. Eventually it 

 disappears, and the long loops of a Hamite, for example, 

 show no trace of it. In many shells called Bacidites, the 

 folded sutures alone distinguish the greater part of the shell 

 from a smooth Devonian Bactrites ; but even the sutures, as 

 may clearly be seen in the specimens from the north of 

 France, are much simpler than in most of the coiled or partly 

 coiled ammonoids. The shell-aperture is oblique, suggesting 

 that the animal had given up swimming for crawling. 



The history of the turretted or helicoid shells is of the 

 same general character. There is, however, reason to suppose 

 that the tendency to this asymmetry usually arose at quite 

 an early period in the life-history instead of coming in with 

 old age as did the uncoiling. Something of the kind was 

 noticed in Palaeozoic nautiloids, and it may be supposed that 

 the tendency was always present, especially in weaker 



