A LAND SNAIL AND OTHER MOLLUSKS 



163 



points upward, the aperture will be on the left. Lymnaa (Fig. 

 92, G) is a very common pond snail. Its shell is coiled in an 

 opposite direction from that of Physa and is called dextral. 



B D 



Fig. Qi. — Marine gastropods. 



A, sycotypus ; B, a nudibranch ; C, the oyster drill ; D, littorina or periwinkle. 

 (From Davenport.) 



Planorbis (Fig. 92, B) differs from Physa and Lymncea in having 

 a shell coiled in one plane like a watch spring. 



Marine Gastropods. — The majority of the marine gastropods 

 have shells, but many of them do not; some of the latter are 



