L. F. Spath— Notes on Ammonites. 117 



asymmetry of no biological importance, would look upon sucli forms 

 as " Psiloceras" abnorme (Hauer), " Psiloceras" Suessi (Hauer), and 

 Oxynoticeras Janus (Hauer), as small forms, with limited geographical 

 distribution, and probably adapted to a crawling mode of life. 1 The 

 first two, in which apparently the asymmetry is fairly constant, are 

 comparable to Psiloceras, with which genus, however, they cannot 

 be united. The last, where the ornament also is affected, has 

 a counterpart in Amaltheus paradoxus (Stahl), though here the 

 suture-line may also be normal, and it seems that Diener's suggestion 

 of a crawling mode of life may hold good for these abnormal forms. 

 In helicoid shells that must be explained in this manner, the position 

 of the siphuncle apparently is not affected, however. 



In a round-ventered shell the position of the siphuncle in the 

 median plane is, of course, not essential; and the functions of the 

 indented septal edge, both as a strengthening feature and as a means 

 of attachment of the animal to its shell, would not be interfered with 

 by an unsymmetrical arrangement. It may be assumed that 

 Psiloceras, though a dominant and thriving genus, was comparatively 

 unstable owing to the instability of its conditions of existence. 

 Neumayr* drew attention to this instability, and pointed out the 

 variability of the Alpine P. (calliphyllum species-group) forty years 

 ago, and Hothpletz 3 shows that even in a locality where there is 

 a continuous transition from the Khsetic Kcessen beds with. Mojsvaritex 

 planorooides to the variegated limestone facies of the planorbis-zone 

 (Marmorgraben, near Mittenwald, Bavarian Alps), only brachiopods 

 and lamellibranchs are found in the first six to twelve feet of Lower 

 Liassic marls. Asymmetry of the suture-line in Psiloceras may then 

 be looked upon as an abnormality that had attained a considerable 

 degree of constancy, but which was due to constitutional instability. 

 This was eliminated when an exceedingly strong radiation with 

 production of keeled and grooved forms (both keel and groove being 

 in part protection of the siphuncle) took place. 



In the case of flat-ventered shells, such as Hoplites splendens, 

 Swinnerton & Trueman think that " asymmetry is probably a growth- 

 phenomenon associated with the tendency of the siphuncle to take up 



Phylloceratids. It may be suggested for e.g. Meneghiniceras and other ' ' Rhaco- 

 phyllites ", that, like many modern marine organisms, they were pelagic in the 

 young and littoral when adult. 



The form figured by Canavari affords a good illustration of the unsatisfactory 

 results of a morphological classification of Ammonites according to the adult 

 suture-line. Canavari wrote : " A remarkable circumstance in this species is 

 the presence of three lateral lobes. Thus it lets itself be grouped in section B 

 of the Amaltheids, according to Neumayr & Uhlig, which comprises the forms 

 with three or more lateral lobes, and perhaps in the sub-genus Sphenodiscus, 

 Meek, with complicated lobes." Inl888, Canavari (" Contribuzione alia Fauna 

 del Lias inferiore di Spezia " : Mem. E. Com. Geol. Ital., vol. iii, pt. ii, p. 34) 

 assigned this form to the genus Oxynoticeras, but its suture-line shows it to be 

 a Ehacophyllitid. 



1 Op. cit., 1912, p. 81. 



2 " Zur Kenntnis d. Fauna d. TJnterst. Lias i. d. Nordalpen " : Abh. k.k. 

 Geol. Eeichsanst., vol. vii, pt. v, p. 25. 



3 "Das Karwendelgebirge " : Zeitschr. d. D. 0. Alpenvereins, vol. xix, 

 pp. 427-8, 1888. 



