1923 CHROSOLOGY 



to species otherwise unnamed. Therefore, in most cases, only one or 

 two species have been figured out of many belonging to a genus : this 

 makes the number of generic names large in proportion to the number 

 of species. But the number of generic names is wholly relative : it 

 depends on the length of time which was taken for the deposition of the 

 Jurassic rocks and on the richness of the faunas which have been preserved. 



All recent researches tend to show that the time-interval required for 

 the deposition of the Jurassic rocks must be one of very great duration — 

 multiphcation of former estimates by tens or, possibly, by hundreds 

 must be made. The old idea that a bed of, say, five feet crowded with 

 layers of Ammonites represented quick deposition — a catastrophic over- 

 whelming by mud bringing about a sudden entombment — is now proved 

 to be wholly erroneous. When the different layers of such a bed are 

 traced laterally across country they are found to thicken out into i,ooo 

 or more feet of strata — a multiplication by 200 ; and that may be only 

 the beginning of such discoveries in regard to lateral expansion. So that 

 instead of a bed crowded with Ammonites being regarded as a case of 

 quick deposition, it has now to be looked upon as an instance of very 

 slow deposition — a^'rich fauna accumulated owing to extreme paucity 

 of sedimentation. 



Twenty generic names given to similar-looking forms from a thin 

 bed of supposedly two dates may seem excessive ; but when investigation 

 of other areas shows that the number of dates has to be multiplied by 

 ten, so that the number of generic names given to contemporaneous 

 species has to be divided by ten, the case assumes quite a different aspect. 



A student with some two or three hundred Jurassic Ammonites 

 may, if he find that each specimen should bear a different generic name, 

 be inclined to criticize the number of generic names as excessive. But 

 he is wholly incompetent to express an opinion on such poor experience. 

 British Jurassic Ammonites have to be studied in their thousands — 

 so rich is the fauna, so great is the number of beds into which the Jurassic 

 strata have to be divided and so limited in certain cases are the exposures 

 of particular dates. Possibly, if all the collections of such Ammonites 

 in the British Islands were placed together, they would be proved to be 

 incomplete by the next month's systematic collecting — some exposures 

 of known richness have hardly been touched, so short a time were they 

 open, so long have they been closed. Possibly, such collections would 

 not represent anything like the full tale of Ammonite species once 

 entombed in the British Islands, for there is reason to suppose that many 

 beds have been removed entirely, and in other beds all the specimens 

 have been destroyed by chemical action. . Certainly any one collection, 

 large though it may be, quite inadequately represents even the collected 

 fauna of British Ammonites. 



Critics of the number of generic names of Ammonites, merely on 

 the ground of their number, should bear such conclusions as these in 

 mind, and they should also remember that generic names are given to 

 record facts — it may be that in the case of two similar forms the suture- 

 hne of one is florid, while that of the other is simple ; or it may be merely 

 that, in similar case, in one form the external lobe is longer than the 

 superior lateral, while in another the reverse obtains. But these details 

 are shown in the plates ; also, they are sometimes noted in the legends. 

 A critic of generic names will readily grasp these details before making 

 his criticisms ; but if he argues that these details are insufficient to 

 justify generic names, then it can only be repHed that he has no idea 

 of the richness and variety of the Ammonite fauna, nor any conception 



