part I] jueassic chronology : lias. 89 



It has, then, to be considered that the specimens as found fossil 

 do not properly represent the conditions of existence, from various 

 reasons which it may be advisable to set out in the following 

 manner : — 



1. Fossilization failure — specimens may be damaged and reduced 



in size before entombment, or may be incompletely fossilized 

 from various causes. 



2. Preservation failure — specimens may be destroyed after 



fossilization. 



3. Extraction failure — the specimens as extracted may not 



properly represent the specimens as entombed. 



Concerning 1 (fossilization failure), this covers ammonites which 

 have lost body-chamber or more through damage before entomb- 

 ment, as by breakage in connexion with redeposition, or may have 

 lost air-chambers through incomplete fossilization. The loss of a 

 crushed remnant of air-chambers may also be due to extraction 

 failure — the rough method of clay-digging in a brickyard would 

 mean loss of such air-chambers, with the result of only body- 

 chambers being found. Body-chamber specimens may then be 

 regarded as a combined result of fossilization and extraction 

 failures. 



Preservation failure (2) is to be distinguished from fossilization 

 failure, in that it is the loss of specimens after entombment, as, 

 for instance, from chemical causes. But this could not be pleaded 

 as a reason for faunal failure, if contemporaneous species of the 

 same family were preserved. 



Extraction failure (3) is a common cause in regard to ammonites 

 in making the collected specimens unrepresentative of the entombed 

 specimens, and still less of the living specimens. In reality, it is 

 of course not so much extraction failure as imperfect extraction. 

 In working with a hammer at the face of a section the extrac- 

 tion of complete ammonites becomes almost impossible, with the 

 result that only the inner whorls are recovered — especially if 

 the specimen be one with whorls not greatly overlapping. Thus 

 the extracted and figured species of Pseudoqrammoceras from the 

 Cephalopod Bed of the Cottesvvolds do not in many cases represent 

 the full size of the living species by a whorl or more l : destruction 

 of the outer whorl before fossilization is in some cases the cause of 

 this, but destruction of it during extraction is also in a great 

 measure responsible. 



To these causes of incorrectness in regard to fossil evidence may 

 be added another which has already been mentioned — namely, 

 zonalization failure (p. 78), attribution of the fossil to a date 

 other than that of the living specimens, either from inaccurate col- 

 lecting, incorrect stratigraphy, wrong record of stratum, mistakes in 

 labelling, and various other accidents with which paleontologists 



1 See S. S. Buckman, ' Monogr. Inf. Ool. Amm.' pi. xxxiii, fig-. 17, pi. xxxiv, 

 fig. 3, <fc pi. xxxv, fig. 4, for examples 



