84 



PALAEONTOLOGY. 



loped in the egg. The spiral turns of the shell w, w are called 

 "whorls," the last w,.ac being the "body-whorl." The lines 

 or grooves formed by their junction are the " sutures," s, s. 

 The "whorls" above the body-one form the "spire" of the 

 shell, pc to a. 



As a general rule, the spiral univalve, if viewed in the 

 position in which its inhabitant would carry it were it moving 

 forwards from the observer, is twisted from the apex down- 

 wards from left to right, the spire being directed obliquely 

 towards the right : but in a few genera, e.g., Glausilia, Physa, 

 the shell is twisted in the opposite direction when it is 

 called "reverse" or "sinistral;" some individuals of Bulinus, 

 Partula, and Pupa, and a few marine species, e.g., Fusus sinis- 

 trosus, are sinistral. The part around 

 which the spiral cone is wound is 

 termed the "columella, o ;" it is ex- 

 posed by removal of part of the shell 

 in fig. 25. This central pillar is some- 

 times simple, sometimes grooved, 

 sometimes plicated ; in some shells it 

 is solid, in some hollow, as in Solarium 

 and Dolium, where the narrow elliptical 

 aperture of the columella is seen to 

 the left of the wide shell-aperture ; it 

 is termed the umbilicus. In Solarium, 

 as in Philippia, the apex of the shell 

 is inverted, and can only be seen by 

 looking into the umbilicus. 



The wide aperture which forms 

 the base of the spiral univalve is 

 bounded by an "outer-lip" (fig. 25, pc, 

 ac) and an "inner-lip ;" the latter offers a smooth convex sur- 

 face, over which the foot of the Gastropod glides to reach the 

 ground. In many univalves, including most vegetable-feeders, 



