52 PROF. H. H. SWiyXEKTOy JlSD MR. A. E. TRl'EMAX [vol. Ixxiii, 



considerably displaced from the median line. These features of 

 the suture-line are only an external manifestation of an enlarge- 

 ment of one half of the septum i hg. 1) at the expense of the 

 other : in such a manner that the internal as Avell as the external 

 lobe has been pushed to one side, and the enlarged half has also 

 been pushed behind the general level of the septum. That the 

 asymmetry is not due to an injury seems ])robable, since it deve- 

 loped slowly enough to permit of the secretion of at least two 

 septa. Xeither does it api)ear to have been due to a change of 

 the conditions which directly affect the form of the suture, since 

 all the details are preserved. The cause was possibly pathological. 

 The facts already discussed show that the form of the septum, 

 even in these asymmetric examples, is independent of the internal 

 organs in tlie neighbourhood of the septum : otherwise a diseased 

 condition of some paired organs (for instance, the gonad) might 

 have been postulated. As it is, the cause must be looked for in 

 a diseased condition of the stretched membrane : namely, the 

 septum-secreting area of the mantle. If this became hypertrophied 

 on one side, it would still assume the form of a stretched mem- 

 l)rane, but being more resistant to pressure from behind, would not 

 become so concave forwards as the other half. This is precisely 

 the condition which the shaj^e of the abnormal septum suggests. 



Usually the change in the position of the siphuncle takes ])lacc 

 slowly and in one direction only : but in a specimen of Hoplites 

 aurltus (PI. IV, fig. 8) it moves from side to side repeatedly, in 

 association with the alternate develo})ment of tubercles on opposite 

 sides of the venter. The mantle apparently sank into the cavities 

 of the tubercles, and thus caused a displacement of the adjoining 

 parts of the body as well as of the mantle. 



Lateral displacement of the siphuncle in tlie example of Peri- 

 sphinctes which we have just described has been accompanied by 

 wholesale enlargement of one half of the septum, but usually only 

 the ventral features of the suture are affected. Thus in the 

 septum of Iloj)Iifes sphndcns (PL IV, fig. 4) all the elements of 

 the internal series and those of the external series which are dorsal 

 to the first latei'al lobe, are quite normal. Further, in the septum 

 of Psiloceras planorhis (PI. IV, fig. 9), the displacement is of a 

 character similar to that observed in Ho2)Jites sfjiendens : for there 

 also the dorsal suture is not a-ft'ected, while the elements of the 

 venter are disturljed even more markedly than in the latter form. 



Thus the displacement of the si2:)huncle in Perisphinctes and 

 KopJifes aiiritus is different in origin from that of Psiloceras 

 and Hoplites splcndeiis. Dr. J. F. Pompeckj ^ has. for example, 

 stated tliat the ty})e of asymmetry in Psiloceras is of systematic 

 value. The discovery, also by him, of a specimen of TragoplufUo- 

 ceras with a similarly asymmetric suture-line, has been mentioned 



' ' Beitriif^e z\\ eiiier Eevisiuii der Aimuoaiiteu dcs Schwiibisclieu Jura ' pt. i 

 (1894) pp. 56-71. 



