18 INTRODUCTION. 



spire runs upwards from the right hand to the left, Plate III. 

 fig. 13. 



In some of the more depressed species of Helix, or Nautilus, 

 great attention is requisite in order to ascertain which is really 

 the upper side of the shell, for it is on that side the spiral turns 

 are to be taken from the centre or apex ; and, in most instances, 

 this is to he determined by the oblique direction of the aperture 

 to the under part, where the lip rarely extends so far as on the 

 upper part. In fixed shells, such as Serpulse, there is no difficulty, 

 as the side which is sessile must be considered as the base or 

 under part. Thus in the Serpula Lucida the fixed part is some- 

 times very small, and the mouth protends spirally upwards, in a 

 contrary direction to the sun ; and therefore must be considered a 

 reversed or heterostrophe shell, the same as if the volutions nearest 

 the mouth had turned laterally upon the centre or fixed ones. 

 This shell, indeed, is most frequently found with regular lateral 

 volutions ; and though subject to great variety, with respect to 

 contortions, it invariably turns the aperture one way. 



In some species of Nautilus, however, there can be no rule to 

 ascertain whether the shells are dextral or sinistral ; for when the 

 aperture is exactly central, the lip embraces the body equally, and 

 the sides of the shell are similar. In others of that genus, as in 

 N. Beccarii and Beccarii perversus, two shells, the principal dis- 

 tinction of which is the contrary turn of their volutions, it is 

 easily determined by the convexity of the upper side, and, of 

 course, the aperture being placed somewhat beneath. 



Chambers are the cavities divided by partitions, at regular or 

 irregular intervals; as in the Nautilus. Plate III. fig. 11, ww 



In some of the Serpula there are also divisions, but they are 

 not regular as in the Nautili ; and besides, they differ from them 

 in being devoid of a siphunculus or communication between the 

 chambers, the animal forms a complete partition, and adds to its 

 shell, which it would appear to be necessitated to do from its body 

 growing too large for its abode. 



Several of the Patellse have chambers formed of laminous par- 

 titions, subspiral cells, or processes ; these in general lie horizon- 



