148 PALEONTOLOGY 



fore advisable to recognize three orders, and to one of 

 these no name having reference to gills can be applied 

 since gills are not preserved in the fossils. 



The three orders may be briefly characterized thus, 

 but it must be understood that as the two other orders 

 diverge from the ancestral order Nautiloidea, there are 

 transitional forms which may not conform strictly to the 

 definitions. 



1. The Nautiloidea are the ancestral and most con- 

 servative order. Commencing as orthocones at the end of 

 the Cambrian period, they pass through the intermediate 

 stages to the sphaerocone in the Palaeozoic era, but sub- 

 sequently remain without further change down to the 

 present time. Only a very few forms, and those quite 

 early, show any tendency to uncoil. 



Generally distinctive features are (i ) the protoconch is 

 not calcareous and is nearly always shed ; (2) the septal 

 sutures are either simple circles or they become slightly 

 wavy ; they are never frilled ; (3) the siphuncle is usually 

 near the centre of each septum, very rarely is it in contact 

 with either margin ; (4) the septal necks point away 

 from the body-margin ; (5) fhere is a hyponomic sinus. 



2. The Ammonoidea contrast with the steady-going 

 Nautiloidea: they had a short life and a merry one. 

 Diverging from primitive Nautiloidea at the end of the 

 Silurian epoch, they hurry through the early stages of 

 coiling, and in the Mesozoic era begin to uncoil. Some 

 of them uncoil completely as early as the late Triassic 

 period ; others at various stages in the Jurassic ; others 

 after some uncoiling start coiling-in again ; but in the 

 Cretaceous period uncoiling becomes very general and is 

 followed by the abrupt extinction of the order (though 

 coiled forms also persist to the last). 



General distinctive features are more difficult to state 

 than in the case of the Nautiloidea, because of the great 



