540 MOLLUSCA SUB-KINGDOM VI 
complicated by the development of secondary inflections on the sides, termed 
marginals. 
Paired or zygous lobes and saddles (Fig. 1092) appear between the two 
azygous lobes and belong to two series, the laterals or externals, and the 
dorsals. The first broad external lateral inflections, called the “first pair of 
lateral saddles,” are formed by the ventral lobe and the corresponding first 
pair of dorsals by the formation of the antisiphonal lobe; and between these 
there appears a broad lobe, either wholly or the most part external (Fig. 
1092, A). This is the stage marked by four lobes and saddles—two azygous 
and two zygous lobes, and four zygous saddles. The wide lateral lobes in the 
next stage (Fig. 1092, Ch) are divided by saddles that arise on or near the lines 
of involution. These divide the two lobes into four, one pair being in part or 
wholly dorsal, and becoming eventually the first pair of dorsal lobes; the 
others develop into the “first pair of lateral lobes.” There are accordingly 
six lobes and six saddles at this stage. In the next stage (Fig. 1092, 4, bm, 
Cm) the saddles bridging the lines of involution become divided by lobes 
arising on or near the lines of involution, and the inner arms of the saddle so 
formed thus become the second dorsal saddle, while the outer form the second 
lateral saddles ; but in some forms they may both pass into the lateral series. 
This stage, therefore, has eight lobes and eight saddles—three paired lobes 
and four saddles on either side of the mesal plane, and two azygous lobes. 
Additional inflections arise in like manner along or near the line of 
involution during succeeding stages. But there is considerable irregularity in 
their advent even in the eight-lobed stage, and still more so at later periods ; 
hence the above description must be regarded as a very general one, although 
serving to indicate a few primitive lobes and saddles that are generated during 
the younger stages, and are usually recognisable in the adult. 
In subsequent stages additional inflections arising on or near the lines of 
involution pass outward as the sides of the shell broaden by growth; and the 
wt 

Fic. 1098. 
Pinacoceras Metternichi, Hauer sp. Keuper ; Someraukogel, near Hallstadt, Austria. Left suture-line, much 
reduced, showing auxiliary (inner) and adventitious (outer) inflections. The three longest lobes in the middle 
are the first, second, and third laterals (after Hauer). 
same law holds true for the dorsum, but of course here the inflections pass 
inward toward the mesal plane. The number of inflections on the dorsum is 
more limited in all forms than the laterals, and they have simpler outlines. 
The inflections added to the sides after the first two or three saddles and lobes 
appear are usually called the auviliaries (Figs. 1096, al, 1098), but the current 
use of this term is not consistent with the development of the inflections, and 
the distinctions are based for the most part on the contrast in size between 
the saddles and lobes as they appear in the adult of different types. When 
the ontogeny is known, however, the auxiliary inflections can be properly 
