On the Measuring of Shells, 209 



valve is the upper or dorsal one, while the other is the lower or 

 ventral; this last being usually furnished with an appendage, as- 

 suming various forms in different species, for the support of the 

 parts of the body of the animal. When the shell is placed on the 

 lower valve with the hole or gap towards the observer, the sides 

 of the shell will consequently correspond with his own. 



^ 3. On the Measuring of Shells. 



Connected as a correct idea of the dimensions of a shell neces- 

 sarily is with a just view of its position, it cannot be wondered 

 at, that confused notions on the latter subject should have led to 

 errors in the former; and hence in the same manner as the back 

 of a shell has frequently been termed the front, the length has 

 been called the breadth, and vice versa. Some authors, indeed, 

 have even gone beyond this, and have introduced a still more fer- 

 tile source of error, by measuring one way in one shell, and another 

 way in another ; thus in one and the same work, describing that 

 diameter as the breadth of one shell, which they assume to be the 

 length of another. This inconvenience will be most securely 

 avoided, by considering as the lengihyihe interval existing between 

 the head and the tail, or that part of the body, which, dividing it 

 longitudinally, is most distant from the head : the breadth being 

 the distance from side to side; and the depth being the space 

 intervening between the dorsal and ventral sides. Thus, in mea- 

 suring* the symmetrical conical univalves, the length will be from 

 the front to the back of the shell, the breadth from side to side, 

 and the depth from the base to the apex of the cone. In the 

 spiral univalves^ the length will be estimated from the back of 

 the aperture to the front, and the breadth across the broadest 

 part from side to side, at right angles with the mouth ; the axis 

 of the shell being measured from the apex of the spire to the 



* A pair of callipers, such as are used by turners and gunners, (or I have 

 lately had a pair made to fold up for the pocket), will be found the most con- 

 venient instrument for this purpose; the measurement being read off in parts of 

 an inch in shells below a moderate size ; for which purpose, I have a quadran- 

 ■ gular rule, whose sides aie divided into 8, 10, and IStlis of inches, and the 

 Frenclj millimetres. 



