582 NORTH AMERICAN INDEX FOSSILS. 



Rather large, somewhat depressed, inflation less pronounced 

 than in preceding species and nearer to larger end, suddenly de- 

 creasing. 



Chickasawan of Alabama, Nanjemoy and 

 Aquia of Maryland, etc. 

 15. C. thallus (Conrad). (Fig. 801, b.) 



Miocenic. 

 Slightly curved, smooth and polished, 

 swelling greatest below middle, regularly 

 tapering in both directions, then more rapid- 

 ly to small end. 

 Fig. 801. a,Cadulusc Chesapeakean of Maryland, Virginia, 



ruptus ; 6, C. thallus. (Both . __. . r _ 



after Md Surv ) ater Miocenic of North and South Carolina 



and Alum Bluff, Florida. 



Class Gastropoda (Snails). 



The gastropods or snails are marine, fresh-water or terrestrial 

 mollusks, with a well-developed head which usually bears tentacles, 

 eyes, ears and a mouth furnished with a file-like lingual ribbon 

 or radula. Ventrally the animal is provided with a muscular 

 " foot " which is generally a broad expansion and sometimes very 

 large but may be variously modified. The visceral organs are gen- 

 erally protected by a calcareous (rarely horny) shell secreted by 

 a mantle. The shell may be conical or saucer-shaped, but is 

 generally coiled into a spiral, either right-handed or left-handed, 

 the former being the more common type. In all cases the shell 

 may be considered a gradually widening cone, which is generally 

 coiled around an imaginary axis which is often hollow. 



The shell begins with the minute embryonic whorls of the proto- 

 conch, which in many types is quite distinct from the rest of the 

 shell. From this portion, which forms the apex of the spire, the 

 coils or whorls of the shell gradually increase in size. Normally 

 the whorls are circular or elliptical in section, but from compres- 

 sion and other causes a variety of forms results. The spire may 

 be high or low, broad or slender according to the mode of enroll- 

 ment, the apical angle varying accordingly. The whorls may rest 

 loosely upon one another or embrace to a greater or less extent up 

 to the middle or ambitus, or even beyond, so that the earlier whorls 

 may be largely or wholly covered by the later ones. When an 



