204 INTRODUCTION TO CONCHOLOGY. 



The order is divided into four families — Tuhispiracea, 

 Macrodomata, Capxilacea, aud Fismracea. 



Tamily I. Tubispiracea. 



What have we here ? 

 Two twisted shells, yet varying, as best suits 

 Their indwelling mollusks. 



Shells of this family may be described as tubular, and 

 irregularly twisted into a loose spire, with a slit occasionally 

 running throughout its entire length. The animal is fur- 

 nished with a horny operculum. 



Two genera belong to this family — Siliquaria and Ver- 

 metus. 



The shell of the genus Siliquaria is tubular, somewhat 

 cylindrical, and irregularly twisted ; the posterior end is more 

 or less attenuated and loosely produced into a spire, which 

 becomes unrolled towards the anterior end, the extremity 

 being open, and the fissure, which is sometimes a little 

 articulated, runs throughout its entire length. 



Little dissimilarity prevails between the shell of the 

 Vermetus and that of liis brother, except that the interior of 

 the shell is often divided by transverse septa. The aperture 

 is round and simple ; the operculum horny and concave. 



