22 NORTH AMERICAN INDEX FOSSILS. 



nites, in which the suture never passes beyond the ceratitic stage 

 (see Figs. 1486-1490). 



The suture of an ammonite septum may be divided into an exter- 

 nal and an internal {dorsal) part. The division is at the point of 

 involution or at the umbiHcal shoulder. The external suture has 

 in its center the unpaired siphonal lobe or ventral lobe, which occu- 

 pies the center of the outer part of the whorl (venter). This lobe, 

 absent only in the early stages of the most primitive genera, is 

 modified in the more specialized types by the appearance of a 

 saddle (ventral or siphonal saddle) in its center. This saddle may 

 be in turn notched or even deeply divided, while a new saddle may 

 appear. The whole series may be modified by the secondary incis- 

 ion (marginals) of the arms of the ventral lobe or the division of 

 the siphonal saddle, or the sides of the lateral saddles. This is the 

 ventral system of the external suture. On either side are the 

 paired saddles and lobes of the lateral system, those on opposite 

 sides of the ventral system corresponding in character and com- 

 plexity. The first is the ventro-lateral or superior-lateral saddle 

 which bounds the ventral lobe; this is followed by the superior 

 lateral lobe; then follows the second or inferior lateral saddle, and 

 then the lobe of the same name. This is the full number in the 

 more primitive forms, but in specialized types additional lobes and 

 saddles appear between the second lobe and the umbilical margin. 

 These appear progressively next the umbiHcal margin (margin of 

 involution) and are called auxiliaries, and are numbered progres- 

 sively towards the umbilical edge. In some cases the lateral saddles 

 divide in a very definite order by the formation of lobes in their 

 centers. Thus a complicated series of lobes and saddles arises, 

 which can only be understood by a study of the individual devel- 

 opment and for which a special nomenclature has been devised.* 



The dorsal or inner part of the suture consists of an unpaired 

 dorsal or antisiphonal lobe. This is entire in primitive species 

 and in the young of specialized types, but becomes bifid or even 

 trifid in the adults of the latter. In some cases, however (retarded 

 or phylogerontic genera), the dorsal lobe remains entire in genera 

 of comparatively late geologic occurrence. On either side of the 

 unpaired dorsal lobe are the members of the paired saddles and 



*See Noetling, Fritz, "Die Entwickelung von Indoceras baluchistanense,^ Geolog- 

 ische und PalcEontologische Abhandlungen, Koken XII., Heft. I, 1906. 



