244 AN INTRODUCTION TO THE STUDY OF FOSSILS 



ship to one another and to living forms is based largely on 

 considerations of shell outline and ornamentation, i.e. on the 

 presence or absence of such shell characters as columella, umbili- 

 cus, callus, etc. The operculum is commonly of corneous nature 

 and is hence rarely preserved in the fossil state. For a consider- 

 ation of the protoconch see page 240. 



Busycon, with its twisted nerve connectives and separate sexes, 

 is a type of one of the two sub-classes of the gastropods, the 

 Streptoneura ; while Clio, with its untwisted nerve connectives 

 and united sexes, is an example of the Euthyneura. Under the 

 Streptoneura belong Pleurotomaria, Bellerophon, Fissurella and 

 Turritella, wMe under the Euthyneura, Clio and Teutaculites 

 are included with the order Opisthobranchia, Helix with the 

 second of the two orders, the Pulmonata. Gastropods are 

 known from the Cambrian to the present, though in abundance 

 only since the Ordovician. 



Derivation of name : Greek gaster, stomach, + pons (pod), ' 

 foot. The stomach is directly above the creeping foot in most 

 gastropods. 



Pleurotomariidae (Fig. 105). Ordovician to present. 



In this family of gastropods the outer lip of the aperture is cut 



by a slit resulting from a corresponding notch in the mantle. 



Through this posterior notch are 

 expelled the digestive waste and 

 the respiratory water ; the incoming 

 water and the food are thus kept 

 "^*-" unpolluted. From the slit in the 

 shell, a slit band, marking the pro- 



FiG. 105. — One of the Pleuro- gressive closing of the slip during 



r^l^^su^IZt:^ m. the growth of the shell, extends 



lings, from the Stones River backward ; around this the growth 



formation (Lower Ordovi- ,. i i i_i* i at r 



cian) of Illinois. ./., slit in ^^cs bend obliqucly. Name from 

 outer lip; st.b., slit band. Greek pie lira, Side, Siud I omari OH, 3. cut. 



^^^ "^ Though an abundant fossil form, 



with over iioo species known, there are but five living species. 



