168 PAL/EONTOLOGV 



constant through the various stages of development, and 

 is a good guide to the affinity between different species. 

 This course is termed the radial line of the species, and it 

 bears a close relation to the form of the apertural border. 

 Thus, if there is a rostrum and lappets without any con- 

 striction behind, the radial line will be more or less 

 sickle-shaped or falciform (Figs. 46, /, h; 47, d), but if 

 there is a constriction it will be straight (Fig. 47,7')- A 

 rostrum without lappets gives rise to a radial line like an 

 inverted L, or Greek F (gammi-radiate, Figs. 39, a ; 45, d, e), 



The ribs sometimes cross the periphery quite straight 

 (Fig. 46, h, K) ; more often with more or less of a forward 

 bend, owing to the general presence of a rostrum (Fig. 

 45, /). The absence of a forward bend in the ribs does 

 not, however, prove the absence of a rostrum (Fig. 46, a) ; 

 it may mean that rostra were produced at intervals and 

 resorbed. In many forms the ribs are interrupted on the 

 middle of the periphery, either by an elevated keel (carinate 

 periphery, Fig. 39, b) or by a median groove (sulcate 

 periphery, Figs. 45, d; 46, i) or by a keel flanked by 

 grooves (carinati-sulcate) . 



The radial ornament is in some families crossed by 

 longitudinal lines. 



The classification of the ammonoids is by no means in 

 a settled state yet, and it is necessary to have some 

 acquaintance with the development of ideas on the sub- 

 ject. A hundred years and more ago, all ammonoids 

 whose shells were normally coiled with the whorls in 

 contact were given the one generic name Ammonites, 

 while those which showed peculiar modes of coiling 



