106 



THE GEOLOGIST. 



septa^ may always be applied in determining the limits of 

 species. 



Dependance of the number of the lobes on the greater, or 

 less envelopment of the whorls. — If we find the number of 

 the lobes very variable, according to the more or less rapid 

 growth of the species, it is much less so, when we compare 

 it to the envelopement of the whorls by each other, or rather 

 to the size of the whorls themselves. In fact, having 

 examined all the species comparatively in this respect, I 

 find that : — 



1. — In those Ammonites whose whorls are simply in 

 contact, as {A. subfimbriatus, Honoratianus, quadrisulcatus, 

 striatisulcatus, strangularis and Juilleti) their number is 

 invariably at a minimum. These species have^ on each 

 side, the two primitive lobes (the superior, and inferior 

 lateral), 



2. — In those Ammonites, of which about the fourth of 

 each whorl is concealed, (A. Cryptoceras, Duvalianus, Lyelli, 

 varicosus, regularisj the lobes are generally three on each 

 side, that is to say, the two primitive lobes, with one aux- 

 iliary lobe 5 they vary nevertheless as far as the maximum 

 of five. 



3. — Amongst the Ammonites, in which only half of the 

 whorls is visible in the umbilicus, (A, Leopoldinus, Paran- 

 dieriy interruptus, tuberculatus, Micheliniamis, mammillaris, 

 Gevrilianus and aiiritus), the lobes vary from three to six, 

 the limits being already greater. 



4. — Amongst those Ammonites, where only a quarter 

 of the whorls is apparent in the umbilicus, [A. fortisulcatus, 

 impressus, splendens, Beaudanti^ Belus, latidorsatus, Guet- 

 tardi, Quercifolius, &^c.), the number of the lobes, varies from 

 five to nine, the immber evidently increasing with the 

 greater envelopement of the spire, 



5. — Lastly, in those Ammonites where the last whorl 



