CLASS V 



CEPHALOPODA 



625 



Fio. lir)7. 



Oppelia sferaxj)^ (OpppI). 

 Upper Jura; SolenhotVii. Com- 

 pressed shell with aptychus (a) 

 preserved in living- chamber and 

 distinct impression of shell 

 muscles ()i) (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 

 general statement. 



Opercula. — Plates have been found iw 

 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 formed 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 OMapfychns ; 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 apitychus, and is always of calcareous composition. 

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

 VOL. I ■ 2 s ^ 



Fio. 1158. 



Oppelia suhradiaia 

 (Sowb.). Inferior 

 Oolite ; Dundry. 

 Aptychus in place, 

 closing apei-tnre 

 (after Owen). 



making 



a 



Fio. IIJO. 



Apt yd) U.I lamdkitius 

 preserved as in Fig. 1157. 

 Upper .Jura; Solen- 

 hofnn, Bavaria. 



