30 INTRODUCTION. 



the centre or fixed ones. This shell, hideed, is most fre- 

 quently found with regular lateral volutions; and though 

 subject to great variety, with respect to contortions, it in- 

 variably 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, as 

 in the Nautilus Calcar, which cannot be thus defined. In 

 others of that genus, as in N. BeccariiandN. Beccarii per- 

 versus, two shells, the principal distinction 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, IV w w w w. 



In some of the Serpulse 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 communica- 

 tion between the chambers; the animal forms a complete 

 partition and adds to its shell, which it would appear to bo 

 obliged to do, from its body growing too large for its abode. 



Several of the Patellae have chambers formed of laminar 

 partitions, subspiral cells, or processes ; these in general lie 

 horizontally, and are quite open at one end, as in the Patella 

 testudinaria, &c. of Linnaeus, and the prickly Crepidula, 

 and Chinese Calyptraea, Plate XIV. fig. 16 and 19; and 

 Plate III. fig. 6, to, or funnel shaped, as in the Sipho 

 striatus. Plate XIV. fig. 21. 



Umbilicus is in general a circular perforation in the base 

 of the lower whorl, or body, of many univalve shells, and 

 extending internally in the direction of the axis. This is 

 common 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. tig. 1, u. 



Suh-umhilicated shells are those which have the umbilicus 

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



