L. F. Spath — Notes on Ammonites. 33 



is in direct contradiction to a causal explanation of their develop- 

 ment" ', as it conveys the impression of an inborn racial necessity of 

 a predestined character. 



The ceratitic suture is one type of line that may recur in the 

 Ammonoid history, just as the same types of ornament appear 

 repeatedly between the Devonian and the Cretaceous, owing to the 

 limited possibilities of variation in each character. Hyatt 2 had 

 considered " the multiplication of the principal inflections in 

 Pseudoceratites of the Cretaceous" to be necessitated "in compensa- 

 tion for the suppression of marginals". The pseudoceratitic suture- 

 line also is not by any means always "reduced ", so that its function 

 as a means of attachment of the animal to its shell is not impaired. 

 A form like Indoceras bahichistatmise, Noetling, with 87 lobes and 

 38 saddles or 75 elements in its suture-line, recalls the acme of 

 specialization among Triassic Ammonites. According to its author, 

 this youngest of all Ammonite genera, the well-formed specimens of 

 which occur abundantly in beds that pass without break or un- 

 conformity into the Eocene, shows no geratologous characters. 



Periodic Evolution and Unstable Environment. . 

 It might be suggested that in the evolution of the suture-line 

 "periodicity of elaboration" occurs such as is claimed for other 

 features and as is observed in the Cretaceous Asteroidea by Spencer,* 

 and that after a period of "catagenesis" affectingthe Pseudoceratites 

 in general, there was renewed elaboration in Lybicoceras or Indoceras. 

 There can be no doubt, however, that Pseudoceratites represent a 

 number of independent developments of different normal Ammonites, 

 just as the tendency to attain dissimilarity in form, in response 

 to differences of environment, is shown in many different stocks. 

 This ranges from the Hauterivian Crioceras to the Maestrichtian 

 Bostrychoceras and culminates (morphologically) in the incredible 

 tangle of Nifponites ; but their relationship is confined to a similar 

 benthonic mode of life. Strong adaptive radiation, such as is 

 generally shown in the young stages of a stock, occurs repeatedly 

 during Cretaceous, as in previous, times. "Changes of structure 

 and diversity of life " are probably "directly related to the physical 

 conditions of habitat",, and the "stability of organic forms is in 

 direct ratio to the stability of the conditions of existence ".* As 

 mutation was so continuous during Cretaceous times, the conditions 

 of existence in so far as they concerned the Ammonites cannot have 

 been stable. 



Extinction and Environment. Lime Secretion. 

 The writer favours the view that the disappearance of Ammonites 

 was not due to inherent phylogenetic relations and inability to 



1 J. v. Pia, N. Jahrb. Miner., etc., i, p. 169, 1914, in review of 

 Mr. Buckman's Yorkshire Type Ammonites. 



2 In Zittel-Eastman, Text-book of Pal., vol. i, p. 544, 1900. 



3 "The Evolution of the Cretaceous Asteroidea " : Phil. Trans. Roy. Soc. 

 Lond., set. B, vol. 204, pp. 156-7, 1913. 



4 Joel A. Allen, "The Influence of Physical Conditions in the Genesis of 

 Species" : Smithson. Inst. Ann. Rep., 1905, p. 401. 



DECADE VI. — VOL. VI. — NO. I. 3 



