1923 PALMOGEOGRAPHY 53 



of the magnitude of the task. To obtain such knowledge, he might 

 confine his work to studying one small family of Ammonites or a very 

 limited set of strata, gathering for a few years all the material possible. 

 Then, one may expect, judging from recent papers done by other workers 

 under these conditions, that he would end by finding the genera in 

 " Type Ammonites " insufficient, and would proceed to div-ide them 

 further. He would reaUse that the idea of the number of genera being 

 large, having regard to the vast amount of material seen, is quite an 

 illusion. 



There is a considerable practical bearing in the gi^'ing of generic 

 names to small details of difference, whether of suture-line or ornament . 

 When such details possess distinct names, the memory can retain the 

 differences, which it is unable to do if they be unlabelled. In well- 

 sinkings, borings and such like, the determinations as to the strata 

 pierced and the chances of success have often to be made on ver^' 

 fragmentary' material. Here the advantage of ha\'ing had small differ- 

 ences duly noted is vital — the knowledge leading to correct determination 

 may mean, in the advice given, all the difference between losing or sa\'ing 

 some hundreds of pounds. Certain costly failures should not have 

 occurred had the fact that what seem to the layman no more than the 

 differences between Tweedledum and Tweedledee been named and noted. 



For it must be remembered that the difference between dum and dee 

 and failure to know which is which may mean an error of some magnitude. 

 If dum and dee lay side by side, failure to distinguish them b}- name 

 would not be so important ; but so often they are ^\adely separated — 

 there is repetition of like forms : then failure to note and know the 

 difference may mean so much. It has just been seen how great is the 

 likeness between forms of the Great and the Inferior Oolite, separated 

 vertically by some hundreds of feet. 



The difference between Suspensites of the Minchinhampton Great 

 OoUte and Zigzagiceras of the Inferior Oolite is httle more than the 

 difference between Tweedledum and Tweedledee — at least, it would 

 seem so, if they were submitted as fragments from a boring. But the 

 much simpler suture-Hne and short Li of the higher form — Li may be 

 visible even in a fragment — ^would be sufficient. The suture-Une of 

 Suspensites looks like a suspended bridge of a single arch, that of Zigzagi- 

 ceras like a two7arched bridge with elaborate pillars. 



Pal.s;ogeography 



The map A given in the opposite page is an attempt to illustrate 

 approximately the distribution of land and water in the area of the 

 British Isles during the early Ammonitoidic Period — in part explanation 

 of the remarks made in pp. 16-24. Detailed discussion of this map 

 must be deferred to a later \'olume, when, also, it is hoped to issue 

 further maps. 



The descriptive naming of the seas presents a certain difficulty ; 

 because it can only be temporarily correct. Land movements make 

 changes — converting a channel into a bay, or the reverse. It is possible 

 that the areas shown as sea are, in some cases, not large enough to 

 provide for temporary extensions of submergence. And, possibly, too 

 large an area of Palaeozoic rocks has been marked as land. The absence 

 of Jurassic (and Triassic) strata from Palaeozoic rocks is not necessarily 

 evidence that no Jurassic beds were deposited upon them. How great 



