76 



THE GEOLOGIST. 



curved or straight, of the edges of the shell, as in the group 

 of the Heterophylli (of the Cretaceous formation A. Hetero-- 

 phylli, Velledce, infundibulum, semistriatus, Alpinus, ^c), 

 the Ligati [A. Beaudanti, ^c), the Clypeiformi {A. Clypei- 

 formis, Gevrilianus, Requienianus, ^-c), the Compressi {A. 

 Largylliertianus, Vibrayeanus^ Beaumontianus, ^c). This 

 series of margins appears the most numerous and least res- 

 tricted, as it is found in a great number of species. The 

 margins belonging to it, although approaching those of the 

 Nautili, differ in this respect by being invariably salient an- 

 teriorly, instead of being inflected. 



I name irregular or transient {moment anees)^ those mar- 

 gins, which, instead of being, at all ages, guided by the ex- 

 terior lines of growth, only appear at certain intervals of 

 time, whether they leave, at certain distances, traces on the 

 shell or not. When they leave traces^ they are intermediate 

 between regular and irregular margins, and they then pre- 

 sent an analogy to the Varices of Cassis, Murex, Ranella and 

 Triton amongst the Gasteropoda*. One might suppose that 

 these old margins were points of repose during the growth 

 of periods w^hen the animal would fear less the shocks to 

 which it might be exposed in this state, on account of having 

 a thickened rim, and I am inclined to suppose that these pe- 

 riodical thickenings of the species, coincided with the epochs 

 of fecundation when it was necessary for the animal to pro- 

 ceed to a greater distance from the shore and thereby be- 

 come more liable to injure its frail shell. Whatever may be 

 the cause, I have observed these remains of margins at the 

 points of termination {points d'arrit) in a great number of 

 Ammonites belonging to different groups, but all to Species 

 having a round back, and never in those whose back is keel 



* This has been noticed by von Biich and Sowerby. 



