PHENOMENA OF REGRESSION AND CONVERGENCE 229 



end, from the equally straight Ammonite or Bacu- 

 lites, owing to the line of suture being rectilinear in the 

 first-named, and strongly sinuous and scalloped in 

 the second. The convergence is, therefore, very 

 superficial, and due solely to the impossibility of 

 arranging in any other form than that of a conical 

 rod, a rectilinear series of chambers in the form 

 of a truncated cone and gradually increasing 

 in size. But, in the Ammonites, the convergence 

 is sometimes carried much further, and affects 

 either the general form of the shell or the dis- 

 position of the sides or of the tubercules, or even 

 the type of the sutural line. Among the most 

 remarkable cases, I shall quote the great similarity 

 of external form (discoid, flat, or sharp edged shell), 

 of the Triassic Pinacocerata, of the Oxynoticerata of 

 the Lias, of certain Oppelice of the upper Jurassic, and 

 of the Ccelopocerata of the Cretacean, etc. I might 

 also quote the recurrence of the forms of the Liassic 

 Arietitce (narrow whorls, square and strong transverse 

 ribs, and the ventral region marked with a double 

 furrow) in the Peroniceras tricar inatum. 



Here, again, the study of the sutural line re- 

 mains the criterion of the demonstration of a con- 

 vergence which bears only on external characteris- 

 tics. But, when in cases, doubtless very rare, like 

 that which our contemporary Kilian has just de- 

 scribed with regard to certain Cretacean Ammonites 

 of the Antarctic regions, the convergence rests at 

 one and the same time on the general form and 

 on the characters of the partitions, the explanation 

 of the phenomenon of convergence becomes an 

 almost insurmountable difficulty. 



