966 



PRINCIPLES OF STRATIGRAPHY 



hollowed and bordered by smooth ridges, while the surface is 

 smooth — features characteristic of the adults of certain species of 

 the genus Protengonoceras of the Lower Cretacic. In all these 

 types the sutures show close relationship and increasing complexity 

 with the progressive changes of the form of the shell. At a still 

 earlier stage the venter is flattened without channel, the section of 

 the whorl being helmet-shaped, while the earliest marked stage 

 shows a rounded venter. The sutures of this early stage are very 

 simple, corresponding in general to the adult suture of Devonic or 

 Carbonic Goniatites, which the form of the shell also recalls. As 



b 



Fig. 255. a. Cross-section of the three outer whorls of Stantonoceras 

 pseudocostatum reduced. (After Johnson.) b. Cross-section of 

 the inner whorls of Stantonoceras guadalupce, enlarged. (After 

 Hyatt.) 



the form changes the complexity of the sutures increases until the 

 complex adult suture is developed. The cross-sections of the vari- 

 ous stages are shown in Fig. 255. The early "Goniatite" stages of 

 Schloenhachia aff. chicoensis Trask, a highly developed ammonite 

 of the Lower Cretacic of Oregon, are, according to J. P. Smith, as 

 follows (43:52/): The first suture (immediately succeeding the 

 protoconch) has narrow lateral lobes and saddles* and a broad 

 ventral saddle. The second suture has a small lobe in the center 

 of the broad ventral saddle, which is thus divided. This corre- 

 sponds to the adult suture of the Devonic genus Anacestes, a 

 simple form of "goniatite" which Hyatt considers the immediate 

 radicle of the ammonoid stock. The third and fourth sutures show 



* Lobes are the backward loops of the suture and saddles the forward loops, 

 i. e., those convex toward the mouth of the shell. The ventral border is the 

 outer border of the shell; the space between the inner margins of the whorl is the 

 umbilicus, in which can be seen the earlier coils. 



