part 1] ox THE MOEPHOLOGY OF THE AMMOKITE SEPTUM. 53 



by Spatli {op. cif. 1911-, p. 351), presumably as corroborative evi- 

 dence of the connexions between Psiloceras and TragopJiyUocerasy 

 Similarly, this character has been described in Cosmoceras by 

 Teisseyre." Sayn -^ mentioned that he had not observed a similar 

 feature in Hopliteii, and considered that this showed conclusively 

 that Hoplites was not directly descended from Cosmoceras. It 

 should be mentioned, however, that A. d'Orbigny (1840, p. 222) 

 had previoush' noticed that asymmetry is of common occurrence 

 in certain species of HopJifes — an observation which Ave are able 

 to confirm. 



An examination of sixteen specimens of Psiloceras planorhis 

 from the Lower Lias of Robin Hood's Bay revealed asymmetry in 

 nine cases. Although asymmetry is of common occurrence in 

 Psiloceras, it would appear to be an unreliable character. Further, 

 Pompeckj's figures show that the displacement varies in amount 

 and direction. 



The careful examination of the suture-lines of more than 600 

 Jurassic and Cretaceous ammonites has shown that asymmetry 

 due to a displacement of the siphuncle occurs sporadically in 

 many widely- separated genera. We have noticed it in the follow- 

 ing cases : — 



Psiloceras 'planorhi)i{'^oweYhy) . ^]S'.U.C,,N.M. Robin Hood's Bay. 9 out 

 of 16. PI. IV, %. 9. 



'^ Nonnaniiites'' hraikenridgi {^oyvQxhY). N.M. Mesvil (Yorkshire), lout 

 of 2. PI. IV, fig. 13. 



Keplerites calloviensis (d'Orbigny). N.U.C., N.M. Ashton Keynes. 1 

 out of 3. 



Aspidoceras perarmaturn (Sowerby). N.U.C., N.M. Faringdon. 2 out 

 of 4. 



Hoplites splendens (d'Orbigny). N.U.C., N.M., B. Folkestone; Cam- 

 bridge. 62 out of 112. 



Hoplites raulinianus (d'Orbigny). N.M. Folkestone. 1 out of 2. 



Dactylioceras commune (Sowerby). N.U.C., N.M., B. Whitby, etc. 10 

 out of 30. 



Cosmoceras jaso7i (Reinecke). N.U.C., N.M. Woburn Sands. 5 out of 12. 



The occurrence of asymmetry in so large a number of examples 

 of Hoplites is noteworthy, and fully bears out d'Orbigny's observa- 

 tion. Yet it should be mentioned here that, in 78 specimens of 

 Hoplites lautiis and H. tuherculatus, we were unable to detect 



^ Mr. Spath remarked that he had not observed any cases of asymmetry in 

 the specimens of Tragophylloceras loscomhi from Lyme Regis. Mr. W. D. 

 Varney, however, has found an example of this species showing asymmetry 

 at Old Dalby (North Leicestershire). 



- ' Bin Beitrag zur Kenntniss der Cephalopoden Fauna der Ornatenthone im 

 Gouvemement Rjasan (Russland) ' Sitz. Math.-Naturw. Classe, K. Akad. 

 Wissensch. Wien, vol. Ixxxviii, pt. 1 (1883) pp. 538-69. 



•* ' Ammonites Pyriteuses des Marnes Valangiennes du S.E. de la France ' 

 Mom. Soc. Greol. France, Paleont. vol. xv (1907) p. 65. 



■* N.U.C. = Teaching CoEection, University College, Nottingham; N.M.= 

 Natural History Museum, Nottingham ; B. = Teaching Collection, Birmingham 

 University. 



