132 THE LIASSIC PERIODS. CHAP. 



small differences, especially in the sutures, which might be best 

 understood as local peculiarities of race. Ammonites to be really 

 known as species, 'must be studied with many examples of every 

 age, including the very young and the very old; the change of 

 form in the course of life being often very great and remarkable. 



In considering the large list of species here presented, the student 

 will find himself obliged to gather them into groups of allied 

 forms : when this is done he will find the groups to be on the 

 whole differently related to geological time; so that 'zones'' of 

 ammonitic life appear one above another in place, one after another 

 in time. 



For example, choosing first ammonites which are carinated round 

 their whorls, we may mark three groups and assign to each its 

 geological ' habitat.' 



3. Keel acute simple (falciferi) ..... Upper lias. 



2. Keel serrated (amalthei) Middle lias. 



i. Keel between two furrows (bisulcati) .... Lower lias. 



And these characters are applicable to the north and the south 

 of England, to France and Germany. 



The relation to time is, however, not so absolute, but that 

 exceptions arise which are interesting to consider. 



In the first group, of bisulcate ammonites, occurs one species 

 which belongs to upper lias, viz. Ammonites bifrons. 



The second group has a remarkable recurrence in the middle 

 oolite series, where Ammonites vertebralis is a conspicuous fossil. 



The third group is continued into the sands which cap the upper 

 lias, and into strata a little above them, which constitute the lower 

 part of the inferior oolite of Dundry. Even in the Stonesfield slate 

 we find falciferous ammonites. 



Again, if we take another series of ammonites without keel 

 or furrow on the whorl, we may mark three groups of forms and 

 assign three parallels of geological time. 



3. Discoid with divided ribs (planulati) ... . Upper lias. 



2. Discoid with undivided ribs (capricorni) . . Base of middle lias or upper 



part of lower lias. 

 i. Discoid with nearly smooth surface (planorbes) . Base of the lower lias. 



Tables of this kind are found of great value in making com- 

 parisons between far separated liassic deposits. More than this, 



