THE CEPHALOPODA 161 



in strata in which they occur, and there are even 

 certain Jurassic shales in the Alps which are full of 

 aptychi, but contain no ammonites. As the former are 

 made of calcite, while the shells are aragonite, more 

 easily destroyed by solution, this is not surprising. In 

 one unique example, from the Inferior Oolite of Dundry 

 Hill, Somerset, the aptychus was found in its natural 

 position, closing in the aperture of the shell, so that it 

 evidently acted as an operculum. The two halves of 

 the aptychus were permanently united in Scaphites 

 (synaptychus) ; in the A vietida and A maltheida and some 

 Palaeozoic genera, a single horny plate (anaptychus) 

 served as an operculum, and many ammonoids possibly 

 had no operculum of any kind. 



The septa of ammonoids always show a tendency to 

 folding at the margin, giving rise to more or less com- 

 plexity in the suture-line. We know nothing of the 

 meaning of this folding, puckering, and frilling, but 

 the various resulting patterns of suture-line are of 

 the greatest value in tracing affinities and classifying 

 ammonoids, so that careful attention must be paid to its 

 details. The number of lobes and saddles is extremely 

 small at first (Fig. 47), but increases during the newer 

 Palaeozoic era, some families then having a very large 

 number (Fig. 47, e, /). In the typical families of the 

 later Mesozoic the number tends to keep within certain 

 limits, and to bear a close relation to the degree of in- 

 volution of the shell. The following description applies 

 especially to those genera in which this relation holds, 

 and needs some modification to apply to the Palaeozoic 



ii 



