Monograph of the Gaderopoda. 303 



which directs the convolutions of the spire in that direction. Such 

 shells are called dextral. All fossil shells of this class, yet found in the 

 Cincinnati GrouD, have their convolutions or whorls turned in this 

 direction, and are hence dextral shells. In a few species of existing 

 shells, and in some monstrosities of dextral species, the spire turns in 

 the opposite direction ; these are called sinistral shells. Occasionally a 

 sinistral shell is found among the dextral species of common snails ; it 

 is, however, quite rare. 



In the spiral shells, the progressive winding of the tube produces a 

 more or less distinct central axis, called the columella, which runs from 

 the apex to the base of the shell, where it forms the inner margin of 

 the aperture, from Avhich the animal protrudes \Yhen in motion. The 

 columella is usually hollow, and terminates at the base of the shell with 

 a small opening, called the umbilicus. The last whorl of the shell is 

 called the body xohorl, from its receiving the body of the animal when 

 retracted. The remaining whorls form the spire; and the impressed 

 line which separates the whorls is the suture. The margins of the aper- 

 ture are called the lips ; the outer lip (labrum) forms the convexity 

 of the shell ; the inner lip (labium) is usually formed by the 

 columella, and is hence denominated the columellar lip. The two 

 lips are sometimes continuous and sometimes separated by a notch, 

 which is often, in the siphonated species, produced into a canal. The 

 shells of the genus Fusispira have the aperture prolonged at the base 

 into an extended canal, while the lips of Cyclora are continuous. The 

 junction of the outer lip with the preceding whorl is frequently marked 

 with a notch, for the reception of the excurrent siphon, and the outer 

 lip, at the greatest convexity of the body whorl, is frequently marked 

 with a slit, as in Pleurotomaria and Murchisonia. The margin of the 

 aperture is sometimes termed the peristome, and when the aperture is 

 formed in part by the columella, such part is sometimes called the %oall 

 of the aperture. Prof. McCoy characterizes this class as follows : 



"Nearly all the Gasferopoc/a enjoy powers of locomotion, and receive 

 their name from the gastric surface of the body being flattened into a 

 disk-like foot, for creeping ; they inhabit salt or fresh water, or the 

 land ; they are all provided Avith a distinct head, usuallv furnished with 

 two tentacles, and perfectly formed eyes ; the mouth is provided with 

 contractile lips, and two or three horny jaws ; frequently the tongue is 

 strap-shaped and set with sharp, hooked points, serving either to rasp 

 seaweed food for the phytiverous groups, or capable of boring holes 

 through shells, to reach the food of the carnivorous genera; the mouth 

 opens into a long, winding oesophagus, receiving at the sides the ducts 

 of large salivary glands ; this ends in a large oval stomach ; from 



