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, Ck) 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, A, m, 

 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 



FIG. 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 auxiliaries (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 



