CL.VSS V 



CEPHALOPODA 



625 



Fio. 1157. 



Oppelia steraspis (Oppel). 

 Upper Jura ; Solenhofen. Com- 

 pressed shell with aptychus (a) 

 preserved in living Chamber and 

 distinct impression of sliell 

 rauscles (h) (after Waagen). 



their marginal inflections, 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 complication of 



the marginals in Jurassic Ammonoids, where the 



number of auxiliary lobes and saddles is often reduced 



(Fig. 1253), and the multiplication of the principal 



inflections in Fseudoceratites of the Cretaceous in com- 



pensation for the suppression of marginals (Fig. 1309), are all accounted for 



by this theory. The phylogerontic forms, in which the lobes and saddles are 



sometimes reduced in number, and the 

 marginals are 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 

 Shells in the living Chamber may be taken 

 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 correspond- 

 ing in position to the hood of Nautilus in 

 a number of Ammonoid shells (Fig. 1158). 

 This positive fact, and the obvious fitness 

 of such plates to serve as opercula, lead 

 to the inference that they were for med by 

 an organ similar to the hood of Nautilus, 

 and protected the animal when it was re- 



tracted into the living chamber. When composed of a single piece, the plate 



is called an anaptychus ; in such cases it is invariably carbonaceous, and was 



doubtless horny in the living animal (Fig. 1160). The anaptychus is rare in 



the Paleozoic, and has not yet been found in the Trias, but occurs among the 



Arietidae and Amaltheidae of the Lower Jura. The operculum, when formed 



of two plates, is termed an aptychus, and is always of calcareous composition. 



It is noteworthy that these plates occur uniformly in the same position among 

 VOL. I 2 s 



1158. 



Oppelia subradiata 

 (Sowb.). Inferior 

 Oolite; Dundry. 

 Aptychus in place, 

 closing aperture 

 (after Owen). 



Aptychus Ja mcllonns 

 preserved as in Fig. 1157. 

 Upper Jura; Solen- 

 hofen, Bavaria. 



