part 1] jurasstc chronology : ltas. 67 



And he gives the perhaps unfortunate impression that, from this 

 single instance of Glydoniceras, he comes to a conclusion which is 

 ' undoubted ' about its strictly-limited horizontal distribution ; but, 

 at any rate from his having chosen it, one may infer that he 

 regards it as an excellent example. Unfortunately, it is about the 

 worst example that he could have chosen, for he is not dealing 

 with comparable genera, as they are not of the same date, 1 and he 

 seems to have neglected to consider all the factors involved, as I 

 may have occasion to show at another time. Of course, the hori- 

 zontal distribution of all organisms is limited, more or less ; but the 

 point to be proved in citing the failure of Glydoniceras north of 

 the Humber is whether it is due to stratal failure or to dispersal 

 failure. As I have remarked above, the analogy with the distribu- 

 tion of modern organisms does not hold. The further question 

 arises, what is meant bj^the term 'limited horizontal distribution'? 

 Mr. Spath remarks in the same paper (p. GS) : 



' Polymorphites ... an unstable genus ... of limited horizontal distribution.' 



As Polymorphites ranges from Yorkshire in the north to Wiirtem- 

 berg in the south, about as far as the sea at that time allowed it 

 to range in those directions, for north of Yorkshire deposits have 

 been destroyed or lie buried under the sea — this is not limited 

 horizontal range in my meaning. That term I would apply to the 

 range of Terebratula fimbria and some other Oolite-Marl brachio- 

 pods of the Aalenian of the .Cotteswolds — their range limited to a 

 few square miles of Cotteswold Hills, where the strata happen to 

 have been preserved. Their present geographical range may be 

 assumed to be less than 'their actual range was ; because on the 

 south and east there is stratal failure due to considerable penecon- 

 temporaneous erosion, and on the north and west the same failure clue 

 to post-Eocene or other erosions. But this is what I call ' limited 

 horizontal range,' because in preserved contemporaneous strata of 

 other parts of England these species are lacking. For some reason, 

 these and many other Cotteswold brachiopods of Aalenian-Bajocian 

 times could not migrate even a few miles south and south-west- 

 wards to areas where other contemporaneous brachiopods flourished, 

 which were equally unable to pass to the Cotteswoldian grounds : 

 here the intervention of a land-barrier seems to meet the case. 3 



But, while the range of the Cotteswoldian brachiopoda may be 

 said to be limited, that of their contemporaries in the South of 

 England was not — some of them ranged into France and even 

 farther. Faunal analyses and comparison will yield some very 



1 Clydoniceraa lived before, and Macrocephalites lived after, the great change 

 in Eurasian maritime geography — the great submergence which opened up 

 large areas to marine organisms. Further, the experience gained in this 

 chronological investigation (as my last paper shows) should warn us about 

 accepting MacrocejyhalUeii as indicating only one date — its species may belong 

 to series in sequence like the Echiocerata of the Raasayan, or be instances of 

 faunal duplication like Uptonia or the armatoids. 



2 II, 1, p. 381. 



F2 



