VARIATION IN SPACE IN GEOLOGICAL TIMES 141 



care the numerous Ammonites gathered in the 

 limestone quarries of the Crussol mountain, which 

 rises on the right bank of the Rhone opposite 

 Valence. Among the most abundant genera of 

 Ammonites in this limestone, the genus Neumayria 

 of the Oppeliides group certainly takes the chief 

 place. These are shells much convoluted, the last 

 spiral, almost entirely covering the inner ones, 

 ornamented on the flank with contorted ribs, and 

 bearing on the external border a row of tubercules, 

 in general smooth and rounded. The elegant orna- 

 mentation of these Ammonites is most varied accord- 

 ing to the subject : the umbilic is more or less open, 

 the ribs more or less firm and numerous, and of a 

 more or less sinuous form ; the tubercules of the ex- 

 ternal border are sometimes close together, some- 

 times wide apart, now round, now lengthened in the 

 direction of the spiral. Sometimes they disappear 

 on the fully grown whorls ; at other times they de- 

 velop and take the form of bristling spikes. From 

 these modes of variation, and from some others 

 which it is superfluous to point out, Fontannes 

 has thought proper, in one natural group alone of 

 this genus, that of Neumayria flexuosa, to separate 

 and describe a dozen different species. The indi- 

 viduals of this group are so plentiful sometimes as 

 to touch one another in certain blocks of these 

 limestone rocks. We find there young and adult 

 specimens of all sizes massed together pell-mell. 

 Every paleontologist who has gathered, as I have 

 done myself, Neumayria in the quarries of Crussol, 

 is unable to free himself from the idea that all these 

 individuals must have bred among themselves, 



