194 PSEUDOCERATITES OF THE CRETACEOUS. 
each series is concerned. Subsequently, however, there is more irregularity. 
Thus the second auxiliary is not necessarily the next to become bifid, but 
it is apt to be the next, and the process passes inward, the innermost saddles 
being usually the last to become bifid, but there is great irregularity in the 
first appearance of bifidity or division among the auxiliaries. 
As a rule, however, this irregularity does not occur in the innermost 
saddle, which is often broad and remains entire later than its neighbors 
and is the last to show complications of outline. 
The second and third of the principal saddles in Placenticeras appear 
to reverse this law of progress inward. After the first has advanced to the 
trifid stage, it is the third which leads in complication of outline and the 
second which comes next. This same reversion is found also in the 
principal lobes In this genus the primitive lateral lobe is continuous in 
development with the third lateral, and it is this that first shows the tritid 
division, which is the incipient stage of complication; then the second 
follow, and after this the first. The last two, however, may progress in 
nearly equal ratio. 
The second, third, and fourth auxiliary lobes are apt to follow the 
lead of the first auxiliary in regular succession, but there is variation in this 
respect, as among the saddles. The general law, however, is the same as 
among the saddles, the innermost bemg the last to modify their entire 
primitive outlines. 
In the development of the sutures there are, however, two series to be 
considered, first the products of the development of the first primitive 
lateral saddle of the nepionic stage, and second the products of the inflec- 
tions in outline of the second primitive lateral saddle. These form two 
series of lobes and saddles on either side of the primitive first lateral lobe 
and obey different laws of development. The complication in the lobes 
proceeds from the oldest lobe, the primitive first lateral, outward and 
inward; the complication in the saddles begins with the primitive first 
lateral and proceeds inward when that saddle is not divided in the neanic 
stage. When, however, that saddle is divided into three, there is more or 
less irregularity in the progression in complexity of the outlines of the 
second and third saddles. The outer division or principal first lateral is, 
however, apparently always the one that grows fastest and leads off in 
acquiring a more complex outline, as may be observed in about all of the 
genera noted below. 
