CLASS IV 



GASTROPODA 



517 



Apex 



Siiture 



in the centre of the base, is designated the umbilicus. A true ümbilicus 

 reaches to the apex of the shell ; when confined to the last whorl only, it is 

 called a false umbilicus. An umbilical fissure is sometimes produced through 

 a partial covering of the umbilicus by the reflected inner lip, er by a shelly 

 growth termed the callus. 



The aperture is variable in form, being most commonly oval, rounded, 

 crescentic or half-round, but is sometimes contracted or even fissure-like. Its 

 margin is called the peristome, the outer partof which forms the outer lip, and 

 the part next the columella the inner lip. Some shells have a continuous, 

 uninterrupted peristome, but as a rule the inner and outer lips are discon- 

 nected. The aperture is said to be entire when 

 rounded anteriorly (inferiorly), as in the Holosto- 

 mata \ it is channelled when a basal notch or 

 canal, caused by an inbending of the margin 

 next the base of the columella, is developed. 

 This anterior canal serves for the lodgment of 

 the siphon, as the tube is called which conducts 

 water to the gills ; it may be either straight or 

 recurved, and in the Siphonostomata it is greatly 

 produced, sometimes even exceeding the aper- 

 ture in length. The outer lip may be entire or 

 incised, thin and sharp or thickened, curved 

 outward (reflected) or inward (inflected), even 

 or crenulated, or it may be produced into alar 

 or finger-like processes. It is sometimes chan- 

 nelled by a canal at the posterior border, in 

 which the anal or excurrent canal is placed. 

 The Upper or posterior portion of the inner 

 margin is commonly designated as the parietal 

 wall, in contradistinction from the lower or 

 columellar portion. The inner lip is formed 

 either by the wall of the penultimate whorl, or 

 by a calcareous callus ; like the outer lip and 

 columella, it may bear spiral folds, which in 

 some cases extend backward as far as the 

 apex (Fig. 843), but sometimes are progres- 

 sively absorbed internally. 



The external ornamentation usually consists of impressed lines or grooves, 

 or of elevated ridges, ribs, folds, nodes, spines and the like. The markings 

 are called spiral when they run parallel with the suture, and axial or 

 longitudinal when they meet the suture at right angles or obliquely. Many 

 Gastropods are brilliantly coloured ; some have a smooth or rough, and others 

 a velvety or hairy epidermis. The fossilisation process is usually destructive 

 not only of the epidermis, but of the coloration as well. 



The essential constituent of univalve shells is aragonite, which usually 

 forms a homogeneous, porcelain-like layer. Many families have in addition 

 to this an inner nacreous layer, which is made up of alternating strata of 

 conchiolin and calcium carbonate, running parallel with the inner surface of 

 the shell. The porcellanous material is composed of three distinct layers, 

 each of which is made up of thin laminae, and the laminae in turn of very 



Canal 



Fio. 843. 



Mitra episcojmlis Linn. View of shell 

 sawed through longitudinally, showing 

 columella with folds. 



