84 BULLETIN OF THE 



development of the funnel in Goniatites, which contrasts with the slighter 

 proportional development of this part in Ammonites. 



Leaving the funnel to be more fully described in treating of the struc- 

 ture of the siphon, we can proceed to the dorsal side of the second sep- 

 tum. This suture is precisely similar to the ventral side in the number 

 of the lobes and cells, though these are shallower and narrower, as might 

 be expected from the more confined limits within which they are neces- 

 sarily distributed. The young Ammonite in the second septum, therefore, 

 really possesses, like the simpler forms of Goniatites, — the Nautilini, 

 which almost exclusively occupy the Silurian epoch, — only large lateral 

 lobes, which, with their duplicate dorsals, make four in all, together with 

 one dorsal, one ventral, and two inferior lateral lobes. 



These last occur upon the edge of the whorl, and are therefore fdled, 

 at first, only by single lateral projections of the animal's body. They, 

 by growth, become duplicated on the ventral and posterior sides ; but, at 

 this stage, they are single. The sutures of the second septum, there- 

 fore, possess one pair of median lobes, the ventral and dorsal ; two pairs 

 of superior lateral lobes, and one pair of inferior lateral lobes, making 

 eight in all. 



Six is the number which has always been attributed to the young 

 Ammonites, and also the normal forms of Goniatites, though differently 

 counted. The external lateral lobes were counted by D'Orbigny and 

 others separately from their duplicatures on the dorsal side ; very in- 

 consistently, however, the dorsal lobe, which is plainly only a duplication 

 of the ventral, was enumerated as a separate lobe. In this way observ- 

 ers have found only two external inferior, and two external superior 

 lateral, with one external, and one internal median lobe, making six in 

 all. D'Orbigny was strictly correct, so far as he went, in assigning six 

 lobes to the young Ammonite, though at fault, with the exception of the 

 ventral lobe, in not observing their counterparts on the dorsal side. 



The third and succeeding septa* on the first volution differ only in 

 the fuller development of these same lobes and cells ; on the early part 

 of the second volution in Deroceras planicosta, and on the latter part of 

 the first whorl in Arnioceras semicostatum, however, the increase in 

 breadth of the whorls exceeds the normal increment of growth of the 

 lobes and cells. When this takes place, the inferior lateral lobes no 

 longer occupy their old position on the umbilical border, but retire with- 



* Plate I, Fig. 4. Plate II, Figs. 7, 8. 



