88 BULLETIN OF THE 



groups with more complicated sutures. This additional evidence is not 

 necessary, however, to show that within the Goniatites there is still 

 greater acceleration displayed between the young of different groups in 

 the development of the lobes and cells, than can be found between the 

 young of the latter and the typical Ammonites. This also corresponds 

 with the scope of the modifications observable in the adults of the 

 entire division between the simple Nautilini, and the Genuifracti or 

 Serrati, or the range of variations in the adults of the same species, 

 as exhibited by Goniates retrorsus and others. No such variability 

 of the sutures exists among typical Ammonites, excluding the allied 

 divisions of Ceratites. Thus, in the adults as well as in the form of 

 the young and the amount of involution, we find greater fixity resulting 

 from the acceleration of the development as we rise higher in the type. 

 This continues until senility or the decline of the type again produces 

 something similar among the aberrant uncoiled genera of the upper 

 Jura and the Cretaceous. The fourth, and succeeding septa of the 

 young Goniatites merely increase the divergence from the common 

 type, the rotund ovisac, and first septum, and the history of these 

 later stages will undoubtedly be found to be characteristic of, and 

 highly useful in the definition of special groups. 



The first true septum of Nautilus has exceedingly shallow lateral in- 

 flections of the sutures, and a ventral cell of corresponding curvature. 

 These could hardly be termed lobes and cells, so slight are they, if it 

 were not for the greater intensity of their expression in the second and 

 succeeding septa. A very important feature of this first and all the 

 succeeding septa, is the dorsal lobe. This has been previously noticed 

 by De Montfort * who founded his genus Bisiphites upon this feature, 

 and by other authors, as an adult characteristic of many of the fossil 

 and recent Nautili. Its developmental history has not, however, been 

 followed out, except by Quenstedt.f This observer described only its 

 later stages, and its disappearance in the adult. He appears also not 

 to have noticed that, in the adult, a slight inflection remains in the 

 suture after the lobe in the septa ceases to exist. Barrande describes 

 the sutural lobe and the corresponding conical depression of the septa, 

 but considers that they may be independent of each other. They are, 

 however, so closely connected in the extreme young, that the sutural 



* Conchyl. Syst. 1808. 



t Petrefactenkunde Deutschlands, Ceph. p. 55. 



