MUSEUM OF COMPARATIVE ZOOLOGY. 83 



necessarily exaggerates its transparency as well as the opacity of the 

 siphonal wall. They cannot really be distinguished at their borders, 

 the one from the other, but in every case the true calcareous septum 

 blends with the brown of the wall of the siphonal coecum. This may 

 be the effect of fossilization, since, in other instances, at a later age, there 

 was no difficulty in distinguishing the two at their junctior 



The second septum* has a very distinct, deep, broad ventral lobe, di- 

 vided by a ventral cell and siphonal fissure, deep superior lateral cells 

 with shallow superior lateral lobes and inferior lateral cells equally slight, 

 the inferior lateral cells beginning to appear upon the border of the 

 whorl. Thus, when seen from the front, this septum has an appearance 

 which leads the observer to suppose that it is interrupted-! An abdom- 

 inal view,! however, readily corrects this mistake, especially in section 

 where the continuity of the suture is readily defined, and a lateral view 

 of the succeeding septa shows the divided ridges of the ventral cell very 

 plainly. § These two ridges normally form one cell. The fissure is 

 probably due to the prolongation of the siphonal funnel. This is a 

 collar-like inflection of the septum, directed posteriorly, the circular ex- 

 tremity resting upon the sheath of the siphon. This funnel has a shal- 

 low channel on the ventral side, which is found in Nautilus also. When, 

 'herefore, the siphon is near enough to the shell, this channel divides the 

 central cell ; otherwise, it ceases to depress the septum before arriving 

 at the edges, and leaves the suture entire. || These facts also demon- 

 strate another, of some importance, that the ventral cell is no adventi- 

 tious product of the near approach of the siphon to the ventral side in 

 Goniatites and Ammonites, but an independent characteristic, constantly 

 present in the latter, but absent from many species of the former. This 

 conclusion is substantiated by a specimen, two septa of which are figured 

 below. ^[ No abdominal channel or gutter was present, and the abdominal 

 cell was left entire. 



The species in which the gutter is absent, among the Goniatites, have 

 a simple ventral lobe, as many of the Nautilini, Goniatites discoideus 

 Hall, and Goniatites Patersoni Hall. In some, however, as pointed out 

 by Barrande in Goniatites plebius, the ventral prolongation of the fun- 

 nel deepens the ventral lobe itself. This is due, perhaps, to the extensive 



* Plate I, Figs. 4, 5. Plate II, Figs. 7, 8. § Plate II, Fig. 8. 



t Plate I, Figs. 1, 0. || Plate II, Figs. 10, 0. 



I Plate I, Fig. 2. ^ Woodcut on last page of text. 



