238 PALEONTOLOGY. 



In one family this organ is modified into an oar for swim- 

 ming. 



The head supports from two to six tentacula placed above, 

 but never around the mouth : sometimes they are absent, 

 they perform the function of touch, and probably those of 

 smell and taste. 



The eyes are small, and two in number ; sometimes they 

 are sessile on the head, sometimes they are placed at the 

 base, at the sides, or at the extremities of the tentacula, and 

 sometimes they are absent. 



The nervous system consists of several ganglia dispersed 

 through the body, and of a bilobed brain which occupies the 

 upper surface of the oesophagus ; and this tube is encircled 

 by a nervous ring ; from the ganglia, nerves proceed to the 

 special organs of sense, the foot, branchiae, digestive and 

 generative organs. 



The superior part of the body is covered with a fleshy 

 mantle, which assumes a variety of forms. Some gasteropods 

 are naked, and others have a shell concealed in the mantle ; 

 but the greatest number have for the most part a univalved 

 shell, into which the animal can retire. As this organ is the 

 only part of their bodies preserved in a fossil state, it is 

 important that the student should make himself acquainted 

 with its anatomy. 



We subjoin, in an extended form, an explanation of fig. 165, 

 and a description of the separate parts of the shells there 

 depicted, commencing with the univalves. 



a. Spire (fig. 1) 

 &. Body-whorl 



c. Beak 



d. Base 



e. Canal 



/. Aperture 



g. Labrum, or outer lip 



h. Labium, or columellar lip 



i. Whorls, or Volutions 



Jc. Sutures 



Z. Apex 



m. Back (fig. 3) 



n. Varices 



o. Columella (fig. 4) 



p. Septa (fig. 5) 



q. Umbilicus (fig. 6) 



r. Area, or anterior slope (fig. 7) 



s. Areola, or posterior slope 



t. Umbones (fig. 8) 



u. Hinge 



v. Ligament (fig. 2) 



w. Disk (fig. 7) 



x. Umbo 



y Base, or Ventral Margin 



z. Height (fig. 9) 



o Length 



b' Auricles 



c' Left valve (fig. 2) 



d' Right valve 



e Valves (fig. 10) 



/' Lunule, or anal depression (fig. 2) 



$ Corselet, or escutcheon. 



