136 



ROBERT TRACY JACKSON OX 



saddles and are comparable to lobe 9 of Fig. 120 and lobe 5 of Fig. 110. Lobe 12 is 

 entire and is comparable to lobes 10-11 of Fig. 120, lobes 7-11 of Fig. 110, and all the 

 lobes of Fig. 118. 



As the relations between the several saddles and lol)es in these septa are somewhat 

 complicated, they are tabnlated for clearness. In the table it is seen that the simplest 

 conditions exist in the yonngest .septum, and reading the columns upward it is seen that 

 the later added septa are progressively more complex. Also it is .seen that the individual 

 septum decreases in complexity from left to right, as tabulated; or, putting it in another 

 way, the simplest portion of the septmn is on the inner or dorsal border, and the septum 

 becomes progre.ssively more complex pa.s,sing from this area toward the ventral border. 



Comparative table of fouk septa of Placenticerax jilacenta figured on Pl. 25, figs. 117-121. 



Lytoceuas and Phylloceras. While the specimen of Placenticcras placenta 

 described is especially clear in showing the relations of localized stages in the develop- 

 ment of the septa of ammonites, such relations when ascertained are more or less clearly 

 marked in any ammonite and often in Goniatites as well. In a recent paper Prof. James 

 Perrin Smith ('08) shows in a beautiful study of the development of Lytoceras alamae- 

 dense Smith and Phylloceras onoense Stanton, from the upper Cretaceous of California, 

 that in progressively added septa from the protoconch on a .similar condition exists to 

 that which I have shown in Placenticeras placenta. The lobes and saddles close to the 

 umbilicus repeat the characters seen in the whole septum of early stages, and passing 

 from the umbilicus outward, or ventrally, there is in later septa a localized ontogenesis 

 comparable in general to the ontogenesis presented by successive septa. His paper was 

 received too late to make further use of it than by this note. 



A few figures copied from Zittel illustrate the truth of the fact, that the septum 



