part 4] juKASsic chronology. 399 



same. Yet Walker found eight specimens of the first and one 

 of the second at Bothenhampton. Now, it is fairly certain that 

 these forms occupy a rather low position in the spinatum zone 

 — may even indicate a deposit which is actually of -^YQ-spinatitm 

 date. If so, the explanation of the geographical record is easy 

 — this earlier bed, or some of its contents, is preserved at Bothen- 

 hampton ; and the bed is certainly present at other places, but 

 it and its contents have been lost from the coast-sections. 



On the other hand, Walker notices particularly that not a 

 fragment of JRhipichonella acuta rewarded his work at Bothen- 

 hampton. Considering that it is a not uncommon fossil at the 

 coast-sections in certain cases, and that it is a usual fossil of Marl- 

 stone localities, this is rather remarkable — moi'e especially as what 

 seem to be its customary associates were found by Walker. Here 

 further analyses may be particularly interesting, with this addi- 

 tional reason — that there are two forms oiRh. acuta — a small form, 

 to which the name was originally applied, and a large form. There 

 is reason to think that the small form is not actually the young 

 (the brephomorph) of the large form, but that it is earlier in date — 

 is an earliei" stage (an anamorph) of the large form. If so, small 

 forms and large could be used as chronological indices, the small 

 form possibly marking a fairly early Marlstone (spinatum) date ; 

 while the large form certainly marks a j)articularly late date, 

 although it must be earlier than Hk. serrata, for it occurs where 

 the serrata deposit has been lost. Is it to be assumed that there 

 was no record of either of these two dates at Bothenhampton P 



It is, I think, correct to say that the small foi-m ranges from the 

 Dorset coast to the Hebrides ; but, before such a statement could 

 be made positively, a critical study Avould be necessary : for there 

 must be a true brephomorph of the large form, and this must 

 be so similar to the presumed adult small form that they would 

 only be distinguishable by what would be regarded as very trivial 

 features. This shows why it is so necessary in palseontology 

 to be precise in noting and naming quite small details, if the 

 full value from the different forms is to be obtained, so as rightly to 

 apprehend what they indicate regarding the history of deposition 

 and denudation. If small biological details be passed over without 

 notice, much of considerable chronological value may be lost. 



It follows, then, that if the idea be correct that the two 

 differently-sized forms of JRhynclionella acuta indicate different 

 dates, the finding of the large form may be taken as fair 

 evidence for the later date ; but the finding of the small form 

 is not necessarily good evidence for the earlier date — not unless 

 we work out precisely whether the small form is a brephomorph 

 or an anamor])h, a labour only to be accomplished when the 

 respective distinguishing characters have been properly ascer- 

 tained. Proof that the anamorph and the large form lived at 

 different dates should be found by faunal analysis, as well as by 

 direct observation. The former should reveal areas where the 

 anamorph existed by itself without any trace of the large form ; 



