THE -GASTROPODA 101 



ing sandy bottoms in shallow water. In time they range 

 from at least the Triassic period. They are carnivorous, 

 using their radula to bore circular holes in lamellibranch 

 shells, as well as those of other gastropods, through 

 which they feed on the animal within. Shells thus 

 bored are very common among the fossils of the Crags, 

 and of the Miocene of Touraine. Natica, however, is 

 only one of the genera which bores in this way, and it is 

 itself sometimes a victim to this mode of attack. 



4. Cerithium serratum (Fig. 29) of the famous " Cal- 

 caire Grossier," the Paris building-stone, of Eocene age, 

 is a turreted shell, different from Turritella, not only in 

 its more elaborate ornamentation but in the form of its 

 aperture, the anterior end of which is produced into a 

 well-marked channel, the anterior canal. The shell attains 

 a length of 75 mm. (3 inches). The spiral angle is about 

 20. The whorls are flat-sided ; in the first 10 mm. the 

 ornamentation is weak and then it suddenly becomes 

 strong. It consists in each whorl of an upper row of 

 prominent compressed tubercles, 12 or 13 to a whorl, 

 just below the upper suture ; a row of very fine tubercles, 

 about 30 to a whorl, three-quarters down the side of the 

 whorl; and a row of rather larger tubercles, 25 to a 

 whorl, just above the lower suture. These rows are 

 crossed by fine lines of growth, which on the last whorl 

 are seen to have the form of a reversed S, though on the 

 whorls of the spire the lower limb of the S is concealed. 

 These growth-lines are parallel to the margin of the 

 outer lip. The aperture is pear-shaped (the anterior 

 canal forming the stem of the pear) ; its long axis is at 



