i8 NORTH AMERICAN INDEX FOSSILS. 



is withdrawn into the shell. A calcareous operculum (the Apty- 

 chiis when double, Anaptychus when single) has been found in 

 many Ammonoids. On the ventral side of the head is a distinct 

 muscular leaf rolled into a tube by the infolding of its free edges, 

 the ambulatory funnel or hyponome (Fig. 1230, k). It widens 

 posteriorly and opens into the chamber in which the gills are sit- 

 uated. It often affects the shell, producing a distinct hyponomic 

 sinus (see Figs. 1308 and 13 19) commonly indicated by the course 

 of the lines of growth. The head is further provided with a pair 

 of large eyes (Fig. 1230, s), with a pair of powerful horny beak- 

 like jaws with calcified tips, and with a lingual ribbon or radula 

 armed with numerous rows of plates and hooks. 



Posteriorly the body is rounded and completely enclosed by the 

 mantle, the base of which is prolonged into a thin, fleshy, hollow 

 tube or siphon, which perforates all the septa of the shell and 

 extends to the apex of the initial chamber. This series of per- 

 forations, more or less complicated, constitutes the siphuncle of 

 the shell. 



The animal is fastened in the shell by two oval muscles (Fig. 

 1230, g), which are situated on either side of the animal and are 

 connected by a dorsal and a ventral band of muscular fibers, the 

 annular muscle or annulus. All the muscles leave shallow impres- 

 sions on the shell, often visible in fossils, where their character 

 becomes of systematic importance. 



The shell of the Nautilus is coiled in a single plane, the later 

 whorls being impressed on the earlier ones so as to hide most or 

 all of the preceding volutions. In the latter case the shell is said 

 to be completely involute, and this state generally marks the acme 

 of development in the different evolutional series. When the inner 

 whorls are visible in the central part or umhilicus the shell is said to 

 be umhilicated. In this case the whorls are less deeply impressed by 

 the preceding ones. In the more evolute forms this impressed 

 zone is shallow and in the more primitive members it appears only 

 late in individual development and may never pass beyond a mere 

 flattening. Hyatt has shown that in early Palaeozoic time the 

 impressed zone appears only when the whorls are actually in close 

 contact, and disappears again when in old age the whorls lose 

 their power of close coiling. In the later nautiloid shells, however, 

 the impressed zone appears, apparently by inheritance, before the 



