66 ME. S. S. BTJCKMAN 0~S | vol. lxxvi, 



years ago: I am speaking now more especially of the subplani- 

 costa and associated horizons of the Raasayan ; and a few notes on 

 such interpretation will be given. But, before proceeding farther, 

 it seems desirable to say a few words about faunal failure, ascribed 

 to dispersal failure and not to the very common phenomenon of 

 stratal failure. 



First for notice are Dr. W. D. Lang's remarks on fishes, con- 

 tributed to the discussion of my former paper l : — 



' Ammonites may Lave had very restricted horizontal ranges .... Many 

 marine fishes are strictly limited in their horizontal distribution.' 



The analogy does not hold in any way. All the knowledge 

 possessed about the distribution of fishes would belong to, say, 

 200 years ; the strata of a hemera must be the accumulation of 

 deposits through some thousands of years. It cannot be said that 

 in a freely open sea the present geographical ranges of species of 

 fishes have been, are, or will be permanent during all the time 

 necessary to deposit the strata of a hemera. If in open areas the 

 geographical ranges are only temporaiy, then it can be argued that 

 such temporary phenomena would, when packed aw T ay in the strata 

 of a hemera, make no showdng at all. Deposition must have been 

 so slow in relation to dispersal that a few hundreds of years' 

 difference in arrival at various places would be marked by no 

 appreciable depositional criteria : geologically, it would be syn- 

 chronism. 



Further, the analogy does not hold, because the method of work 

 with the fossils is not concerned with species of merely local distri- 

 bution — a species which ranges from Gloucestershire to Wiirtern- 

 berg, or from Somerset to the Rhone Basin, is not a species of 

 local distribution. Its failure in contiguous or intermediate areas 

 is what has to be explained. In passing from Somerset to the 

 Rhone, or vice versa, a species must have left dead on the way to 

 mark its trail. Why are they not found in Dorset ? Land-barriers 

 come into play in some cases, as will be shown presently ; but they 

 must not be called in for cases like this, where a little later and a 

 little earlier certain ammonite species are found ranging from the 

 Rhone Basin to Yorkshire and even to the Hebrides, but failing 

 in some intermediate areas. And the failure of the ammonites is 

 not the sole evidence ; often there is direct proof of the failure of 

 their strata : thus a bed c with its own fauna is found resting 

 directly on bed a, leaving no room for bed b which is found in 

 neighbouring areas on bed a. 



Next may be noticed a remark by Mr. L. F. Spath in a footnote 

 to his paper ' Notes on Ammonites ' 3 : 



' Compared with the almost universal distribution of the genus Macrocepha- 

 lites, this restriction of Clydoniceras [in the southern part only of the Corn- 

 brash outcrop in England] is interesting, and shows that, like many modern 

 marine organisms, certain Ammonite genera were undoubtedly strictly limited 

 in their horizontal distribution.' 



1 X, 2, p. 326. 2 XIX, p. 31. 



