INTRODUCTION. 15 



served in tlie tubular, curved form, the example of wliicli will 

 be the Dentalium, fig. 12. 



Dentaliiim Elephantinum. 



This has an opening at the anterior termination a, called the 

 aperture. The opening at the posterior end (p) is named a 

 fissure, or perforation. The ribs running along the sides of 

 the shell are longitudinal, or radiating. And the lines roirad 

 the circumference are lines of growth, or concentric — each one 

 having in succession, at earlier stages of growth, formed the 

 aperture. They are described as concentric or transverse. 



Symmetrical Convolute Univalves. 



The Nautilus, the Spirula, the Scaphite, and the Ammonite 

 are the leading types of this form ; but when we use the term 

 symmetrical, in reference to these, the word must not be un- 

 derstood in its strictest sense, for no shell imperfectly symme- 

 trical : but it means that there is no perceptible difference in 

 the proportion of the two sides ; as in the human body, the 

 right side is larger and more powerful than the left, yet to a 

 degree so small that it gives no apparent bias to the figure. 



CHAMBERED SHELLS. 



Many of the shells now under consideration are chambered, 

 that is, the internal cavity is divided into separate compart- 

 ments by plates reaching across it, named Septa; and the only 

 connection between the chambers is formed by the small pipes 

 passing through them, to which the name of Siphon is attached. 



