MOLLUSC A— CEPHALOPODA. 23 



lobes of the dorsal series. These are numbered from the center 

 outward, to the line of involution. The first dorsal saddles bound 

 the median dorsal lobe, one on each side; then follow the first 

 dorsal lobes and the second dorsal saddles in regular order to the 

 umbilical margin, where the new lateral and dorsal sutural ele- 

 ments appear. 



The outlines of the paired lobes and saddles become modified 

 in the later Palaeozoic and the Mesozoic species by the appearance 

 of secondary inflections or marginals. These first modify the lobes, 

 which become bifid and trifid (ceratitic suture), and later on the 

 saddles (ammonitic suture), which may become complexly incised. 



Modification of the Aperture. — In nautiloids the aperture is 

 mostly simple, and modified only by the hyponomic sinus (see 

 ante) on the ventral side. In some specialized groups a contraction 

 of the aperture occurs, often resulting in the formation of narrow 

 slit-like openings or sinuses. The hyponomic sinus is generally the 

 longest, and besides it there may be from two to six lateral or 

 brachial sinuses (see Hexameroceras, Fig. 1378). In addition to 

 the brachial, there is often a median dorsal sinus {Trimeroceras, 

 Pentameroceras, Septameroceras, Fig. 1379). In the Ammonoidea 

 the living chamber is also contracted towards the aperture, in many 

 forms (phylogerontic), and in old-age individuals. Periodic con- 

 striction and thickening of the aperture, and subsequent expan- 

 sion on resumption of growth, produces varices which, from the 

 thickening of the margin, may be represented by grooves on the 

 internal mold (Figs. 1393, a, b, m; i^iy, 1420). In the Palaeozoic 

 Ammonoids a hyponomic sinus is commonly retained, but in the 

 more specialized types and in the later forms this is commonly 

 replaced by a ventral crest, or even by a long projecting rostrum. 

 Lateral crests and lappets also develop in a number of genera 

 (Fig. 1452). 



Modification of the Venter. — In the more primitive coiled cepha- 

 lopod shell the venter is rounded, and unmodified. It may be 

 broader or narrower than the dorsum and may curve to a greater 

 or less radius than that of the whorl as a whole. The modifica- 

 tions are, on the one hand, towards a flattening, and on the other 

 towards acuteness. In the compressed forms it is often sharply 

 acute (Figs. 1489, b; 1491), or it may be truncated by a flat band 

 (Fig. 1403, m), or a channel, bounded by sharp ridges (Fig. 



