476 



MOLLUSKS I GASTEROPODS. 



two, and its fossil allies. They have the sutures of the 



shell simple, and siphuncle central. 



ORTHOCERATiDyE. This Family contains cephalopods 



which have the shell straight, curved, or discoidal. They 



are all fossil, and abound in the Paleozoic Rocks. 



AMMONITIDA:, OR AMMONITE FAMILY. This Family 



embraces cephalopods which 

 have a shell bearing a close 

 resemblance to that of the 

 Nautilidae, but with the su- 

 tures angulated, or lobed 

 and foliated, and the siphun- 

 cle external, that is dorsal, 

 as regards the shell. They 

 are all fossil, and abound in 

 the rocks below the Ter- 

 tiary. The species are nu- 

 merous, and vary from an 

 inch to two or three feet in 

 diameter. 



Ammonite. 



SECTION II. 



THE CLASS OF GASTEROPODA, OR GASTEROPODS. 



THE Class of Gasteropoda comprises mollusks which 

 effect their locomotion by means of a broad muscular 

 expansion called a foot. Some of them are destitute of a 

 shell, but most are protected by a single shell, and are 

 often called Univalves. When first hatched, they are 

 always provided with a shell, but in many families this is 

 soon concealed in the mantle, or wholly disappears. 



Gasteropods, considered in regard to their manner of 

 breathing, may be divided into two groups ; Pulmo- 

 nifera, or Air-Breathers; and Branchifera, or Water- 

 Breathers. The former undergo no apparent metamor- 



