134 ROBERT TRACY JACKSON ON 



in growth (many species of Cardium, Pecten, Lima) , or in which radii multipl}' l)y regu- 

 lar dichotomy, would show only the longitudinal series (Beecher, '98). 



Ammonites. In ammonoid cephalopods septa divide the shell into numerous 

 chamber.s, and the intersections of these internal partitions with the inner surfaces of the 

 shell, called the sutures, are an important feature in classification. These sutures are 

 often highly complicated, but, as shown by many investigators, the young ammonite has 

 simple sutures. The first formed septum either is simple as in nautiloids, the primitive 

 order of the same class, or in the more specialized ammonites has one or two lobes and 

 saddles. In no ammonites have the first formed sutures dendritic or other plications of 

 lobes and saddles, which are so common in the adult. Simple lobes and saddles in 

 ammonites are characteristic of embryos and primitive types. Senile individuals or 

 degradational types also have simpler sutures than their own adults or the progressive 

 types of their own stock. A greater or less degree of simplicity may, therefore, be 

 accepted as a proof of youth, priraitiveness, or degradation. 



In studying the individual septa of any of the more specialized ammonites, it is 

 seen that the more complicated lobes and saddles are usually situated on the outer or 

 ventral portion of the septum (PI. 25, figs. 117-123). Passing from this area inward, 

 or toward the umbilicus, the lobes and saddles become simpler, until close to the 

 umbilicus the simplest lobes and saddles of the septum are found. As stated, simple 

 lobes and saddles are characteristic of the young and of primitive types, and they become 

 progressively more complicated during growth in the individual and the phylum. It 

 appears, then, that in the individual septum of complicated types we have a more or less 

 complete recapitulation of the ontogeny and phylogeny of the type, for as we pass from 

 the umbilicus outward, starting with the relatively simple, there is a progressive com- 

 plication of lobes and saddles. 



Placenticeras placenta, a specimen of Placentlceras placenta (Dekay), from 

 the Cretaceous of Dakota, illustrates the relations between the ontogeny of succeeding 

 septa and the evidence of ontogeny as expressed in localized parts of the individual 

 septum. This specimen was kindly loaned me by Prof. Alpheus Hyatt. It is figured on 

 Pl. 25, figs. 117-121. In Fig. 117 the younger portion of this specimen is represented of 

 life size ; at the sections indicated by numbers 118, 119, 120, 121, are situated the 

 septa which are represented enlarged in Figs. 118-121. 



A relatively simple septum from one of the younger volutions is shown in PI. 25, 

 fig. 118. In this septum all the lobes and saddles are simple, but those toward the ven- 

 tral border are larger and better differentiated. The simple saddles of the whole of this 

 septum are comparable to saddles 7-11 of Fig. 119, saddles 10-11 of Fig. 120, and 

 saddle 11 of Fig. 121. 



