88 ME. S. S. BITCKMAN ON [vol. lxxvi, 



words, the difference of size may not be geographical but chrono- 

 logical. 



For instance, the former correlation of the Gloucestershire strata 

 yielding small tirmati with the Radstock strata furnishing giant 

 armati — the placing of both deposits as armatum zone — in- 

 volved the opinion, implied if not actually expressed in so many 

 words, that the small armati of the former were either dwarf 

 forms of, or merely the inner whorls of, the large forms of the 

 latter area. Now, the stratal sequence of Raasay, combined with 

 results from other areas, has shown that the deposits with small 

 armati are separated from those Avith the large forms by a time- 

 interval expressed in some 300 feet of deposit. 1 In Gloucester- 

 shire are found preserved, not far above oxynotus beds, the strata 

 with the small armati, which are now known to be the lower 

 horizon, while the upper horizon fails. Only a few miles away to 

 the south, but contiguous to the Mendips, at Radstock, are 

 preserved strata (in about the same position with regard to Oxyno- 

 toids) yielding the large armati which are now known to be the 

 higher horizon ; and here the lower horizon has not been preserved. 

 Thus, the difference between these two faunas of small and large 

 ammonites is chi'onological, not geographical, and the difference 

 in the relative preservation may be said to be geological — it is an 

 accident of diastrophism that strata of the date of small forms 

 have been preserved in Gloucestershire, and not the strata with the 

 large forms ; while the reverse happens to be the case in North 

 Somerset. 



This seems to give, in part, at any rate, a clue to the Hierlatz 

 and Spezia faunas ; but, in order to consider them properly, and to 

 compare them with faunas of other areas, it becomes necessary to 

 have a technical nomenclature : because different causes produce 

 the small ammonites which may be represented in palaeonto- 

 logical works or collections. Further consideration shows that the 

 term ' small ammonites ' covers a whole series of phenomena, and, 

 that any investigation of a dwarf or small fauna, which at first 

 sight might be thought a simple proposition, is really quite 

 complicated. 



The condition in Avhich ammonites are found fossil may differ 

 materially from their condition when alive. In opposition to the 

 cases in which specimens occur usually only as inner whorls (and 

 those inner whorls by no means all the air-chambers) are those 

 cases like the Acanthopleurocerates of the Gloucestershire Lias 

 and the capricorns of the Dorset Coast, of which as a rule only the 

 body-chambers are found ; the air-chambers are seldom preserved 

 or, if found attached to the specimens, are crushed flat. Compression 

 to flatness of a greater or less portion of the inner whorls of 

 serpenticones is quite common, and the loss of this part in extraction, 

 leaving only the outer whorls — body-chamber and some air- 

 chambers — is the general result. It all depends on how many of 

 the air-chambers could be filled with deposit before fossilization. 



1 II, 8, p. 267. 



