70 L. F. Spath — Notes on Ammonites. 



quoted in this connexion : " The ratio of increase [of the chambers] 

 is apparently uncertain and is influenced probably by the growth of 

 the animal, which would, of course, depend on the supply of food 

 and other circumstances." 



Mode ok Life. 



In this connexion, again, it may be noticed that among Jurassic and 

 Cretaceous Ammonites the long-lived Phylloceratids and Lytoceratids 

 show the greatest constancy of the septal ratio ; and their stability 

 is further shown by the infrequency of another feature of the suture- 

 line, namely its asymmetry, which will be considered presently. 

 But since reference to the probably nectonic character of these 

 Phyllocerates and Lytocerates and certain oxynote shells has been 

 made, it is necessary to insert a few remarks on Hyatt's hypothesis, 

 that the "progressive complication of Ammonite sutures took place 

 because of their utility in helping to carry and balance the shell 

 above the extruded parts when the animal was crawling". 1 That 

 author believed that the complication was directly correlated with 

 the outgrowth of rostra, and stated: "The presence of a rostrum 

 indicates the disuse and disappearance of the swimming organ 

 (hyponome), which in Nautilus causes the formation of the hyponomic 

 sinus in the aperture and flexed growth-lines on the venter." 



This connexion between the disappearance of the hyponomic sinus 

 (not of the swimming organ) and the complication of the lobe-line 

 seems very doubtful. A form like Beloceras, with hyponomic sinus, 

 was probably as much an adaptation to an actively swimming mode 

 of life as later shells of a similar flat and acute-ventered shape, 

 whether they were Nautiloids like Phacoceras oxystomus (Phillips) 

 and Stenopoceras Rouilleri (de Koninck), or typical Ammonites like 

 Pinacoceras or Aspidoides. Kerr 2 suggested that "the structure of 

 the infolding edges of the hyponome and the muscular character 

 of this organ would enable the animal to unroll and flatten it out so 

 as to be available for crawling". Thus it would be homologous 

 with the foot of Grasteropods. And as Ammonoids gradually adopted 

 a freely swimming mode of life, the hyponomic sinus disappeared. 

 In Beloceras already, it must have lost its original use. Thus there 

 would have been no disappearance of the swimming organ, but the 

 absence of a hyponomic sinus in the later Ammonoids would show that 

 the hyponome was not used for crawling as in typical Nautiloids. 



Hyatt also thought that "the shells of Ammonoids, being less 

 bulky in proportion than those of Nautiloids, were correspondingly 

 less buoyant". But it has already been mentioned that Nautilus 

 has been found attached to the bottom, whereas some of the least 

 "bulky" of Nautiloids, like the above-mentioned Phacoceras and 

 Stenopoceras, were adapted to an actively swimming existence. It is 

 probable that neither the typical Ammonoids and Nautiloids, nor 

 such a radially symmetrical, benthonic structure, as the Devonian 

 " Gomphoceras", already referred to, or e.g. a delicate Carthaginites 



1 Loc. cit. (in Zittel-Eastman, 1900), p. 544. 



2 "Anatomy of Nautilus 'pom'pilius'''' : Proc. Zool. Soc, 1895, pp. 664-86 

 (quoted in Zittel-Eastman, 1900, p. 506). 



