544 MOLLUSCA SUB-KINGDOM VI 
more widely distributed (Figs. 1229-1231). . In their extreme modifications 
they become more or less uncoiled and finally perfectly straight. 
Crick and Waagen maintain that Ammonoids had an annular band as well 
as shell muscles, and that these served both to hold the animal in the living 
chamber, and also formed an air-tight band around 
the face of the mantle, fastening the latter to the shell 
(Fig. 1108). Such was, however, probably not the 
only means of attaching the animal to the shell. The 
steady progressive complication of sutures, affecting 
both lobes and saddles as well as their marginal in- 
flections, is directly correlated with the outgrowth of 
rostra. The presence of a rostrum indicates the 
disuse and disappearance of the swimming organ 
(hyponome), which in Nautilus causes the formation of 
the hyponomic sinus in the aperture, and flexed 
growth-lines on the venter. These facts and the 
hae Pes “Oy ep SL eRATIONs littoral habits of Ammonoids show that 
Upper Jura; Solenhofen. Com- they probably crawled along the bottom with their 
pressed shell with aptychus (@) : : : 
preserved in living chamber and Shells carried above them, very rarely swimming. 
Meee UD datter Waaen), “Their shells are also less bulky in proportion than 
those of Nautiloids, and correspondingly less buoyant. 
All these observations justify the hypothesis that the progressive complication 
of Ammonoid sutures took place because of their utility in 
helping to carry and balance the shell above the extruded 
parts when the animal was crawling. The greater com- 
plication of the marginals in Jurassic 
Ammonoids, where the ales of auxiliary 
lobes and saddles is often reduced (Fig. 
1192), and the multiplication of the prin- 
cipal inflections in Pseudoceratites of the 
Cretaceous in compensation for the sup- 
pression of marginals (Fig. 1224), are all 
accounted for by this theory. The phylo- 
gerontic forms, in which the lobes and 
saddles are sometimes reduced in number, 
and the marginals also less complex— 
together with the position, form, and mode 
of growth of the last volution, and the 
short rostra—suggest that these creatures Fi. 1109. 
could not have been active crawlers during — oOppelia subradiata, 

Fic. 1108. 

oy - Fic. 1110. 
aye or 7 ; Sowb. Inferior 
the gr eater part of their ontogeny. Oolite; Dundry. Aptychus —lamellosus, 
The occurrence of broods of young Aptychus in place, preserved as in Fig. 1108. 
- Nes © closing aperture Upper Jura; Solen- 
shells in the living chamber may be taken (after Owen). hofen, Bavaria. 
as suggesting that some Ammonoids were 
viviparous, but the examples of this are too rare to be relied upon for making 
a general statement. 
Opercula.—Plates have been found in situ closing the aperture and corre- 
sponding in position to the hood of Nautilus in a number of Ammonoid shells 
(Fig. 1109). This positive fact, and the obvious fitness of such plates to 
serve as opercula, lead to the inference that they were formed by an organ 
