18 INTRODUCTORY 



this is to be 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 Serpulae, 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 turns 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. 



Chambers are the cavities divided by partitions, at regular or 

 irregular intervals ; as in the Spirula. Plate III. fig. 11, WW 



w w w. 



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

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

 from them in being devoid of a siphuncvdus 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 Patellae have chambers formed of laminous par- 

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

 tally, and are quite open atone end, as in the Patella Testudinaria, 

 the Crepidula, and Calyptraea. 



UmbUiciis is in general a circular perforation in the base of 

 the lower whorl, or body, of many univalve shells. This is com- 

 mon to most of the Trochi, in some species of which it penetrates 

 from the base to the apex ; widest at the base, and gradually 

 tapering to the top. Plate III. fig. 1, u. 



Sub--umbiUcated shells are those which have the umbilicus 

 covered in a greater or less degree by a thin process ; which, in 

 eome, almost entirely closes the opening or mouth. This charac- 



