MOLLUSC A— GASTROPODA. 583 



angulation occurs, the space between it and the suture above it 

 constitutes the shoulder. The shoulder angle may be simple or 

 keeled, or furnished with nodes or spines. 



The primitive ornamentation of the shell consists of revolving 

 ridges or spirals, and of transverse folds or ribs. Primary spirals 

 appear in regular succession on either side of the first primary, 

 which generally becomes the shoulder angle if angulation occurs. 

 Secondary spirals appear by intercalation between the primary 

 ones, and generally are absent in the young shell, except in some 

 highly accelerated types. Tertiary spirals are intercalated between 

 the preceding groups in more specialized species. Ribs are regular 

 transverse foldings of the shell generally extending from suture to 

 suture. They are usually spaced uniformly and crossed by the 

 spirals. In specialized types when a shoulder angle is formed, 

 they become concentrated as nodes upon this angle, disappearing 

 from the shoulder above and the body below. Spines may replace 

 the nodes in later stages. They form as notches in the margin of 

 the shell and are subsequently abandoned, often remaining open 

 in front. Irregular spines may also arise on various parts of the 

 surface of the shell (see Platyceras). When a row of spines is formed 

 at the edge or outer lip of the shell — this sometimes remains 

 behind as a varix (Murex). Varices may also be formed by simple 

 expansion of the outer lip, and a subsequent resumption of growth 

 from the base of the expansion. These simple varices may project 

 from the shell (Scald) or be reflected backwards (Harpd). Periodic 

 enlargements of ribs (Murex, Cerithiuni) are not to be classed as 

 varices. 



The aperture or peristome of the shell may be simple or variously 

 modified. An outer and an inner (columellar) lip are generally rec- 

 ognized. These may be continuous with each other, or may be 

 divided below by an anterior notch. This, in some types (Fusus y 

 etc.) is drawn out into an anterior canal, of greater or less length. 

 An upper or posterior notch is present in certain (chiefly old age) 

 types, and this may result in the formation of a ridge or shelf 

 next to the suture (Clavilithes). An outer (lateral) emargination 

 or notch, sometimes prolonged into a slit occurs in certain types 

 (Pleurotomidae, Pleurotomaridae, Bellerophontidae, etc.), and the 

 progressive closing of this slit may give rise to a definitely marked 

 slit band. In some cases the slit is abandoned and left as a hole 



