150 PAL/EONTOLOGY 



features when full grown. Tetrameroceras (p. 136) is an 

 instance; Gomphoceras (Ord.-Sil.) is an exogastric form 

 with T-shaped aperture; Phragmoceras (Sil.) an endogastric 

 gyrocone. In Ascoceras (Sil.)> after growing as an ortho- 

 cone to some length, the shell suddenly expanded, the 

 earlier portion was detached, and the few septa that were 

 afterwards formed are so strangely shaped that the gas- 

 chambers are on the dorsal instead of the posterior side 

 of the animal. 



(3) Ophiocones (Fig. 45, a) are found in the Devonian 

 and are abundant in the Carboniferous. Sphaerocones 

 begin with Solenocheihis of the Carboniferous, and Nautilus 

 (Trias. -Rec.) continues this form. Hevcoglossa (Trias.- 

 Cainozoic) and Aturia (Cainozoic) are remarkable as 

 mimicking the Devonian clymenids (see below), both 

 in the folding of their septa and the dorsal position of 

 the siphuncle. 



(4) Uncoiling forms are very rare among nautiloids, 

 and are confined to early times. Lituites is Ordovician, 

 Ophidioceras (Fig. 45, b) is Silurian : both are ophiocones 

 (sometimes beginning as cyrtocones) which revert to 

 orthocone form, the former at an early period of its life, 

 the latter only in old age. Otherwise the Nautiloidea 

 seem to have found stability and permanence in the 

 sphaerocone stage. 



The Ammonoidea are more interesting to the geologist 

 than are the Nautiloidea, because of their great value 

 as zone-fossils, which arises from the wonderful vitality 

 of the stock, which during six geological periods threw 

 off swarm after swarm, each going through an evolu- 

 tionary cycle with rapidity and then either dying out 

 or giving rise to a new stock repeating a similar cycle, 

 until at last exhaustion is reached in the Cretaceous 

 period and the whole order dies out. 



