OHDKH II 



AMMONOIDEA 



543 



Phyllocampyli have retained the hyponomic sinus ; short obtuse crests a pp-ar 

 in the Trias, and continue thereafter. Jurassic and Cretaceous Ammonoidfl 

 have as a rule more pointed rostra than those of the Trias, and frequently 

 develop lateral crests and lappets (Figs. 1103-1107). 



Very decided decrease in the dimensions of the, living chamber during tin- 

 senile stage does not occur as a rule among Palaeozoic forms ; but this con- 

 dition appears among the Triassic Haloriti</<" and Tm/iifii/iH' wit li a ronvspondmg 

 contraction of the aperture. The Arcestidae (Fig. 1164) and some species of 

 Discocampyli also often have very narrow openings during tin- paragerontic 

 sub-stage, but the condition is in no sense phylogerontic except in /,"/>//, 

 and the like (Figs. 1160, 1161). 



Pompeckj, in an important essay, asserts that contracted living chambers 

 are invariably developed in old age, and that small 

 shells possessing them are consequently not im- 

 mature individuals, but dwarfs (Fig. 1107). It is 

 probable that large numbers of shells are indeed 

 dwarfs, but it is also a fact that contraction of 

 the living chamber and volutions occurs in some 

 forms during comparatively early stages ; and 

 sometimes in such a way as to affect the ephebic 

 stages of the ontogeny, when the forms become 

 truly phylogerontic. This latter term is used to 

 designate shells in which the ontogeny has become 

 permanently modified by the assumption of retro- 

 gressive characters that were introduced first in 

 the senile stages of allied progressive species. 

 Whether these peculiar forms have contracted 

 apertures in their earlier stages, and then resorb 

 them before building further, or whether they 

 never add lateral lappets, rostra, etc., as claimed 



by Pompeckj, until the last resting stage of the ontogeny (Fig. 1107), it is 

 obvious that they are permanently affected by phylogerontic characters. 



F , 1103 



mpoioceras cristatum, Deim-. sp. 

 A P erture with v " lillai 



FIG. 1104. 



Sphaeroceras Srong- 

 niort;, Sowb.sp. Oolite. 

 Aperture with broad, 

 contracted ventral ros- 

 trum. 



1105. 



\''iriiiiinii!ti's 



'.'. 1 1. sp. Oolite. 

 Aprrtuiv 



lappete. 



. . 

 with 



Fio. 1106. 

 Glochicera* 

 batum, Opp. sp. 



Jura, 

 lappets. 



Lateral 



Oecoptychius re- 



!' H;i;Ui s]>. 



.Jura. Living chamber 



I'ontr.-ictfd, with m*- 

 tnmi and 



These forms are comparatively rare in the Trias (Lobites, Cochloceras), but 

 their number is sensibly increased in the Jura, although usually confined 

 to special localities. During the Cretaceous they become more numerous and 



