part 1] OS THE MORPHOLOCiY OF THE AINFMOXITE SEPTT'M. 55 



scune keeled forms ; for it lias been recorded in Ammonilps 

 miserahilis Quenstedt, in Oxynoticeras lieteroplenrnm, and in 

 O. guihali. In the latter genus, at all events, the exception is 

 only apparent, for the venter maybe strictly considered as rounded, 

 on account of the hollow keel.^ 



Asyiiimetrv has been observed in Hoj)Iitr,s hiherculatiis by 

 (y. C. Crick, - but here it was accompanied by a distortion of the 

 shell ; yet a specimen with asymmetric ornament may have a 

 normal suture, as seen in an example of Ammonife^ amaUlieus? 



The asymmetry which occurs so commonly in Hoplites sjjlendens 

 and a. raulinianus is usually accompanied by the displacement 

 of the sijDhuncle into the angle bounding the flat venter. At 

 12 mm. diameter in each of the specimens the sutures were sym- 

 metrical, and at this stage the specimens had round venters 

 (PI. IV^fig. f)a). From this stage until a whorl later (PL IV, 

 fig. 5 J) the siphuncle became gradually more displaced ; but onty 

 the ventral part of the septum became distorted, for all the 

 features dorsal to the first lateral lobe remained normal. Similar 

 characters were observed by A. d'Orbign}^^ in the development of 

 asymmetr}^ in Ammonites denarivs. Asymmetry in these cases 

 is probably a growth phenomenon, associated with the tendency 

 of the siphuncle to take up a stable position along the angle 

 bounding the venter. 



Accompanying the displacement there has been in many forms 

 a rapid change in the form of the suture, which is ])robably a 

 result of the increased area on that side of the siphuncle. It 

 illustrates the manner in which the form of a suture may be changed 

 owing to a lupidly increased height of whorl. Thus in Hoplites 

 splendens (PL IV, figs. 4-(5) the external saddle at first has two 

 main divisions ; but later it becomes trilobed. Further, in Cosmo- 

 ceras (PL IV, fig. 10) the external saddle of the enlarged side is 

 practically double the breadth of the corresponding saddle of the 

 other side. In association with this increase in relative size the 

 saddle has become more deeply denticulated, and the major frillings 

 have become indented by numerous minor frills which are but 

 feebly represented on the other side. This peculiarit}^ has been 

 studied in some detail by Teisseyre.-"' 



He suggested that the displacement that we have desei'ibed 

 is a growth-phenomenon associated with the form of the whoi-l. 

 Solger^ believed that the movement of the siphuncle away from 

 the median plane was due to the a(lo])tion of a recumbent habit. 



' A. Hyatt, Smithsonian Contvibntions to Knowledge. No. 073 (1880) pi. x. 

 fig. 28. 



'^ ' A deformed example of Hopllles ti(bercttJat}im,' Geol. Mag. doc. 4. vol. v 

 (1898) p. 541. 



3 O. Fraas, Jahreshefte Wiirttemberg. xix, 1863. p. 111. 



** A. d'Oi'bigny, ' PaL'ontologie Franeaise : Terrains Cr.'tac.'s ' vol. i (1840) 

 p. 222. 



' L. Teisseyre, Sitz. Math.-Naturw. Classe, K. Akad. Wissenscli. Wien. 

 vol. Ixxxviii, pt. 1 (1883) pp. G04^()08. 



•' F. Solger, ' Beitriige zur Geologic von Kamerun,' 1904. 



