542 



MOLLtlSCA 



SUB-KINGDOM VI 



in proportion to the volution, and apt to be nearer the centre (Figs. 1 100, 1101) 

 during the young than at later stages, and is also monochoanitic, as in Nauti- 



FIG. 1100. 



Tropites, cf. Phoebus, Dittm. Trias. En- 

 larged section in the median plane of the 

 young, showing monochoanitic funnels in 

 nepionic stage, then transitional, and later 

 chloiochoanitic funnels, a, Position of pro- 

 toconch (after Branco). 



Paltopleuroceras spinatum, Brug. Lias. 

 Section parallel to median plane, show- 

 ing position of the siphuncle (after 

 Branco). 



loids. It remains monochoanitic (having only a funnel) in the Gastrocampyli, 

 Microcampyli, and Mesocampyli, as well as primitive forms of Eurycampyli ; but 

 it becomes diplochoanitic (having both funnels and 

 forwardly directed collars) in more specialised Car- 

 boniferous Eurycampyli, and finally chloiochoanitic 

 (funnels lost, collars alone remaining) in Permian 

 genera. Most Triassic and all Jurassic and Cre- 

 taceous genera have the siphuncle chloiochoanitic. 

 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 

 gowb consequently organic deposits are not found in the 

 camerae of these shells. 



Living Chamber. This varies greatly in all of 

 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 Mesocampyli, except during the gerontic 

 stage of some species when a ventral crest arises, as demonstrated by Clarke. 

 In the Eurycampyli 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 



FIG. 1102. 



Parkinsonia ParMnsoni, 

 Middle Jura. Median section show- 

 ing siphuncle with bulbous enlarge- 



ent (c), prosiphon (p), and 



sition 



of protoconch (), (after Munier- 

 Chalm 



Chalmas). 



