ORDER II AMMONOIDEA 543 
Phyllocampyli have retained the hyponomic sinus ; short obtuse crests appear 
in the Trias, and continue thereafter. Jurassic and Cretaceous Ammonoids 
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 the 
senile stage does not occur as a rule among Palaeozoic forms; but this con- 
dition appears among the Triassic Haloritidae and Tropitidae with a corresponding 
contraction of the aperture. The Arcestidae (Fig. 1164) and some species of 
Discocampylt also often have very narrow openings during the paragerontic 
sub-stage, but the condition is in no sense phylogerontic except in Lobites, 
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 Fic. 1108. 
apertures in their earlier stages, and then resorb _ Dipotoceras cristatwm, Delue.- sp. 
them before building further, or whether they Gault. Aperture with ventral 
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. 


Fic. 1104. Fic. 1107. 
Sphaeroceras  Brong- Fic. 1105. Fra. 1106. Oecoptychius re- 
niarti, Sowb. sp. Oolite. Normannites Braiken- Glochiceras nim- Jractus, de Haan sp. 
Aperture with broad, ridgi, Sowb. sp. Oolite. batum, Opp. sp. Jura. Living chamber 
contracted ventral ros- Aperture with lateral Jura. Lateral contracted, with ros- 
trum. lappets. lappets. trum and lappets. 
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 
