INTRODUCTION. 



21 



Spire of non-symmetrical Univalves. 

 In counting the whorls of which the spire consists, we com- 

 mence at the apex, and reckon downwards to the last, or body 

 whorl. The spire is described as being long or short in rela- 

 tion to the aperture : in which case, all that is above the aper- 

 ture is measured with the spire. Its apex requires particular 

 notice, as the character of the whole shell frequently depends 

 upon the particulars observable in this part, It is sometimes 

 obtuse, or blunt ; sometimes acute, or sharp. In the Cones it 

 is frequently flat, and in Planorbis it is concave. It is some- 

 times of a different structure from the rest of the shell, retain- 

 ing the horny and transparent appearance which characterized 

 it when the animal was first hatched. The Tritons present 

 an instance of this, although it is not always observable, owing 

 to the tenderness of the substances which causes it to break or 

 fall away in many specimens. A very remarkable instance 

 also occurs in Bulinus decollatus (cut, fig, 27, 28), so named, 

 because the apex, to the depth of several whorls, falls off, and 

 the shell is decollated. In this, and many more instances, 

 among Pupseform land shells, the occurrence of this circum- 

 stance seems to be by no means rare or accidental, a provision 

 having been made for filling up the opening by a septum. A 



B1 W ^ 



Fig. 23, obtuse; 26, acute ; 27 , 28, decollated ; 29, concave ; 30, papil- 

 lary ; 31, mammellated; 32, discoidal. 



