544 



MOLLUSCA 



SUB-KINGDOM VI 



FIG. 1108. 



Oppelia steraspis, Opp. sp. 

 Upper Jura ; Solenhofen. Com- 

 pressed shell with aptychus (a) 

 preserved in living chamber and 



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 

 gregarious littoral habits of Ammonoids show that 

 they probably crawled along the bottom with their 

 shells carried above them, very rarely swimming. 

 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 number 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- 

 geroritic 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 

 could not have been active crawlers during 

 the greater part of their ontogeny. 



The occurrence of broods of young 



FIG. 1109. 



Oppelia subradiata, 

 Sowb. Inferior 

 Oolite; Dundry. 

 Aptychus in place, 

 . . closing aperture 



shells in the living chamber may be taken (after Owen). 



FIG. 1110. 



Aptychus lamellosus, 



preserved as in Fi-. I his. 

 Upper Jura ; Solen- 

 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 



