48 SINISTRAL SHELLS. 



thus far studied. You notice in nearly all the cuts 

 that when the apex is uppermost, as it should be, the 

 aperture is on the side next to your right hand. All 

 such shells are said to be right-handed or dextral 

 shells. Our little Trifoi^is^ however, has the aperture 

 upon the left side and is said to be left-handed or 

 sinistral. As this little brown shell is less than one- 

 fourth of an inch long, a picture would have to be 

 magnified a good deal to show its form plainh', but 

 you will see examples of sinistral shells in Fig. 107 

 and Fiof. 108. There are some whole Qrenera of mol- 

 lusks which have sinistral shells, particularly the 

 Physas, which are the fresh water snails so common 

 in all little brooks. In some few species part of the 

 specimens are dextral and part are sinistral, but, as a 

 rule, left-handed shells are rare and quickly excite 

 remark. 



Cerithiopsis tuberculata^ Mont., Se-rith-i-op'-sis tu- 



ber-cu-la^-ta, is much like the last species in its 



general appearance, though it is larger and its shell 



is dextral, as shown in Fig. 29. The spire 



consists of six or seven whorls, the spire is 



tuberculated or covered with little projections, 



and the sutures are conspicuous. The color is 



Fi^2q. dark brown, and the length is from one-fourth 



to one-half of an inch. 



Cerithiopsis cohimna^ Cpr., col-um^-na, has a shell 

 of ten whorls ; in form it is slender, its sutures are 

 inconspicuous, and "the nodules are close, like 

 strung figs" (Cpr.). Its color is light browm, and its 

 size is the same as that of the last species. 



A southern species should also be mentioned, 

 Cerithiopsis assimilata^ C. B. Adams, as-sim-i-la'-ta. 

 In size and color it is like the last species, but it is 



