542 MOLLUSCA SUB-KINGDOM VI 
in proportion to the volution, and apt to be nearer the centre (Figs. 1100, 1101) 
during the young than at later stages, and is also monochoanitic, as in Nauti- 

Fic. 1100. 
Tropites, ef. Phoebus, Dittm. Trias. En- 
larged section in the median plane of the eee To 
young, showing monochoanitic funnels in Paltopleuroceras spinatum, Brug. Lias. 
nepionic stage, then transitional, and later Section parallel to median plane, show- 
chloiochoanitic funnels. «a, Position of pro- ing position of the siphunele (after 
toconch (after Branco). Branco). 
loids. It remains monochoanitic (having only a funnel) in the Gastrocampyli, 
Microcampyli, and Mesocampyli, as well as primitive forms of Lurycampyli ; but 
it becomes diplochoanitie (having both funnels and 
‘forwardly directed collars) in more specialised Car- 
boniferous Hurycampyli, and finally chloiochoanitie 
(funnels lost, collars alone remaining) in Permian 
genera. Most Triassic and all Jurassic and Cre- 
taceous genera have the siphuncle chloiochoanitie. 
The above stages are repeated in regular succession 
during the ontogeny of chloiochoanitic forms (Fig. 
1100) except when accelerated development (tachy- 
genesis) occurs, and then the monochoanitic stage 
may disappear. The reduction in size of the 
siphuncle among Ammonoids is obviously corre- 
lated with loss of functional importance, as is also 
the case among more specialised Nautiloids ; and 
EE Medel Soh aa Rr consequently organic deposits are not found in the 
Middle Jura. Median section show- camerae of these shells. 
Se Maa Coy. aid pete Living Chamber.—This varies greatly in all of 
Bh apiigennteh: £0) (after Munier- its dimensions, thus indicating differences in the 
size and proportions of the animal, since its body 
parts were probably wholly contained within this cavity. The lines of growth 
and the few apertures known among Microcampyli and Gastrocampyli show that 
they had hyponomic sinuses on the venter, and were swimmers like Nautiloids. 
The same was probably true of the JJesocampyli, except during the gerontic 
stage of some species when a ventral crest arises, as demonstrated by Clarke. 
In the Lurycampyli and Glossocampyli many species that retain the so-called 
goniatitic form have hyponomic sinuses, but occasionally short ventral crests 
appear, and later these become general. Only radical Palaeozoic forms of the 

Fia. 1102. 
