IN GENERAL. 683 



downwards, and turned the ligament forwards. BLAINVILLE, on 

 the contrary, described it in the position which we have indicated 1 . 



In the univalves (cochlea, testae univalves, subbivalves) the fol- 

 lowing parts may usually be distinguished. The extremity of the 

 spire is called the point (apex), which, when the animal is alive and 

 in motion, is turned upwards and backwards. Ordinarily the cavity 

 of the shell terminates in a point at this part, but it is sometimes 

 truncated horizontally (apex decollatus s. truncatus), which must not 

 be mistaken for an accidental fracture which always leaves an open- 

 ing. In some univalve shells (as in the genus Patella), the cavity 

 between the point and the aperture is neither twisted to the right or 

 left, nor forwards or backwards. In most, on the other hand, this 

 cavity is turned. Usually all the turns run obliquely from above 

 downwards (turbinated shells, cochlea turbinata et turrita)', yet in 

 some the wreaths or turns run from left to right in a transverse 

 direction, whereby the last wreath includes the rest externally (testa 

 convoluta s. involuta, the involute shell, as in the genera Conus and 

 Oliva), whilst others again are turned in one and the same vertical 

 plane, from behind forwards and from above downwards (cochlea 

 revoluta, as in the genus Nautilus). 



The opening (apertura) of the shell is opposite to the point. 

 That part of the cavity, which is visible within the mouth, is called 

 the throat (faux). The mouth may be excised with a sinus ante- 

 riorly (apertura emarainata). Whenever it terminates forwards in 

 a channel (apertura canalifera), then the tube is called the tail 

 (cauda s. rostrum}. At the mouth the external margin (labium 

 exterius s. labrum] is distinguished from the internal margin (labium 

 internum s. margo columellaris) . The spindle (columella) is that 

 part which runs in the middle of the shell directly from the point to 

 the aperture, and round which, as an axis, the wreaths are turned. 

 Such a calcareous axis is not present in all univalve shells; to see 

 this part distinctly, a shell sawn longitudinally must be provided. 

 At the internal margin of the aperture there is often on the ex- 

 tremity of the spindle, especially in the shells of younger molluscs, a 

 cavity which is called navel (umbilicus). Shells, in which this aper- 

 ture is wanting, are named imperforate (cochlece imperforatce). 



1 That which with LiNN^iUS is the right and the left shell still preserves thia 

 name, since he turned the bivalve not only upside down, but also the fore part 

 backwards. 



