PLACENTICERATID A. 193 
In all genera having but one principal lateral (Hoplites, Schloenbachia, 
Buchiceras), this lateral is a direct and closely connected modification of the 
primitive lateral of the young. Also in genera having two or three saddles 
these are not preceded by an intermediate stage in which one large lateral 
arises like that of Schloenbachia and Hoplites splendens, but on the contrary, 
these principal laterals arise through the continuous growth of the two or 
three primitive and more or less tongue-shaped and at first entire mar- 
ginals that develop in the top of the primitive first lateral lobe, as has lately 
been shown by J. Perrin Smith. 
When these are arrested in development and do not increase in com- 
plexity and remain entire or become simply bifid or trifid, etc., the differen- 
tiation between them and the auxiliaries is lost and the sutures of the adult 
are similar to those of the Engonoceratide and the like. When, however, 
these do acquire more complex outlines and the development is duly pro- 
gressive, its effect is invariably and naturally greater upon the saddles and 
lobes of the outer side, which have free growth and are most called upon 
to help hold the animal in the shell. These, either on this account or for 
some other reason as yet unknown, certainly during development as a rule 
become more complex than the inner laterals. This complexity is, asa rule, 
less in direct proportion to-the distance of the saddle or lobe from the 
periphery if an allowance be made for the natural division due to the greater 
or less persistence and sometimes more or less independent development of 
the largest lateral lobe. This is the primitive first lateral in normal forms, 
but in retrogressive genera it may be, as in Placenticeras, the third marginal 
lobe of the primitive first lateral, as has also been shown by Smith. This 
greater development can be accounted for by the greater stress of the muscles 
at these median parts in balancing the shell while crawling. It interferes 
with the regularity of the gradations in size and complexity of the series, 
and marks the division between the principal or larger lateral lobes and 
saddles and the so-called auxiliaries. 
In most groups of Ammonitinze the first lateral becomes bifid and the 
centran marginal lobe is the first that appears. Usually the next saddle to 
become bifid is the first auxiliary. This occurs in Placenticeras, while the 
second and third laterals still have entire bases. In other words, similar 
laws govern the development of the two series of saddles, the principal 
laterals and the auxiliaries, so far as the development of the first saddle of 
MON xLIV—03—-13 
