MOLLUSCA — CEPHALOPODA. 
173 
then returns for a longer distance, and ends straight after ^tallery 
yet another bend (Fig. 96 e ) ; and Baculites, in which, after a Table-eases 
small initial coil, the shell continues straight to its aperture i4 & 15. 
right up to death (Fig. 96 g). By study of the 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 
Neocomian shells in the Crioceran stage are ribbed in the 
irregular manner characteristic of Lytoccras ; see, for example, 
C. villersianum in the Table-case. Those named Fictctia 
Asiicriaim also show the peculiar Lytoceran suture. In 
many of those from the Speeton Clay, on the other hand, 
e.g. 0. quadrat.um, the ribbing is very ii-regular, and appears 
to have been derived from such a form as Perisphinctes or 
Holcostcphamis. The same appears to be the case with 
the large Aptian Crioceran JBowcrhanhi and its relative 
Macroscaphites grandis, the earlier whorls of which resemble 
those of the Portlandian Ilolcosteplmnus 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 h ) ; 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 Haraite, for example, 
show no trace of it. In many shells called Baculites, the 
folded sutures alone distinguish the greater part of the shell 
from a smooth Devonian Betetrites ; 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 Paheozoic nautiloids, and it may be supposed that 
the tendency was always present, especially in weaker 
