250 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Helix (Fig. no). Eocene to present. 



This is a pulmonate gastropod. The mantle cavity contains 



no gill and itself functions as a lung for breathing air, since its 



walls are traversed by blood 

 vessels. The animal is 

 hence terrestrial in habitat. 

 The genus and its sub- 

 genera include more than 

 4000 species. 



T . Sketch side view show- 

 ing aperture and spire, label- 

 ing spire, aperture, whorls. 



2. What modification has 

 the animal undergone that 

 enables it to live upon land ? 



Fig. 1 10. — The common land snail, Poly- 

 gyra albolabris, from New York State. 

 ( X f .) This is closely allied to Helix. 

 Dorsal view of creeping animal, showing 

 the coiled calcareous shell and the active 

 feelers or tentacles ; each of the two longer 

 tentacles terminates in an eye. Note that 

 the coil of the shell is posterior (compare 

 with Fig. 113). (Redrawn from Simpson.) 



CLASS D, SCAPHOPODA 



The Scaphopoda are characterized by an elongate, bilaterally 

 symmetrical body which thus looks worm-like. This body is 

 inclosed in a tubular, calcareous shell which is curved and taper- 

 ing, and open at both ends, and is secreted by the mantle lobes ; 

 the body is attached to the posterior part of the shell by muscles. 

 The concave side is dorsal. The larger end is anterior ; through 

 it the foot can be protruded and used for burrowing in the sand 

 and mud. In this end of the shell, likewise, is the short proboscis 

 or " head," which bears the mouth. 



The structure of the body is very simple. There are no special 

 circulatory organs, no gills, respiration being effected by the 

 whole mantle, and the head bears no eyes. Into the posterior 

 aperture of the shell is drawn the water used for breathing, and 

 through it is discharged the body waste. The food is chiefly 

 Foraminifera. 



The somewhat similar tubular shells of gastropods and 

 cephalopods may be distinguished by the fact that they are 

 alwavs closed at one end. 



