PLACENTICERATID2. 195 
The tendency of the saddles to become bifid in their first stage of 
complication was noted by Branco and since by Nicklés, but in the lobes 
this varies. In these the first stage of complication is apt to be either bifid 
or trifid, according to the form of the entire lobe of the preceding stage. 
If these be pointed, the next step in complexity is the formation of a trifid 
top, if they be rounded or flattened, the next grade is usually a bifid 
termination. 
In the genera described below there is a notable tendency toward the 
formation of trifid lobes in all of the outer lobes and in a number of the 
auxiliary series, the inner and last auxiliary lobes showing a tendency to 
become bifid. 
The saddles and lobes in any one suture of the genera which have 
arrested development of the lobes and saddles, and even in some like 
Sphenodiscus and Placenticeras with very complex outlines, show a graded 
series of modifications from the line of involution outward. These have 
frequently such simple entire lobes and saddles near the umbilicus that one 
can see at a glance that they are like the entire outer lobes and saddles of 
the young. The suture, in fact, presents a series of modified forms that 
show in a general way the history of the development of any one of the 
outer lobes or saddles, if it be traced from its entire stage to the suture 
which is being observed. This is due to the fact noted above and also 
shown in Branco’s observations and the author's on the young, viz, that 
new lobes and saddles as a rule are added from the line of involution so 
that these in any extended suture line are younger or later introductions. 
In some genera with arrested development these remain comparatively 
unmodified, but in most genera of Ammonitinze and in Placenticeras these 
do become modified and have complex outlines in later stages, although 
never so complex as in the outer saddles and lobes. When this modifica- 
tion by development takes place these internal and younger lobes and 
saddles proceed or develop by repeating the stages passed through by 
their outer and more rapidly developed companions of the same sutures. 
In other words, the lobes and sutures of Ammonitinz exhibit the same 
law of repetition or parallelism in local development which was. first 
discovered by Dr. R. T. Jackson among the Echinoids. He there showed 
that a newly introduced plate of the corona passed through stages of 
modification in the course of its subsequent growth which were parallel 
