MOLLUSC A. 9 



follows : First by a discolored area {bs) , laid down by the 

 mantle in front of the scar, all around the posterior end and 

 sides, but not at the anterior end. That this pigment was 

 deposited in narrow bands just posterior to the great 

 muscle, and on its sides ; that this deposit accompanied 

 a series of layers made by the mantle border (so), which 

 occupied precisely the spaces between the horny layers 

 (black lines sk in Fig. 14) ; that these interspaces were 

 periods of growth, and the horny deposits periods of 

 rest, from growth ; that the muscle remained stationary 

 during the period of rest, and consequently here a deeper 

 furrow was built up, and the spiral changed its direction. 

 Between the periods of rest, or as the shell was building 

 itself outwards, the muscle followed, stopping to rest at 

 short but irregular intervals. The manner of the advance 

 even can be determined by these scars, since the central 

 posterior part bulged out first in the direction of least re- 

 sistance, and was followed by the sides. 



The marks of this progress are the striations (sfr) show- 

 ing the course of the muscular bundles as they were crowded 

 onwards, and the cross-ridges (s') left by the anterior edge 

 of the muscle, which enable the observer to show in some 

 cases its former position and shape. 



This preparation is made by filing on the outer side of 

 the shell, to an appropriate depth, and then striking it 

 smartly with a hammer. The inner deposits usually flake 

 off when this is done, and leave the track of the muscle 

 exposed, as in Fig. 13, showing the long, shallow furrows, 

 the tracks of the bundles of muscles, the transverse furrows 

 and ridges ; showing the anterior border of the muscle 

 when taking a comparative rest (probably" in winter), and 

 the bow formed by the sides and posterior border at the 

 same time. Here, also, it is proper to notice a fact of 

 great value in the comparative study of shells ; namely, 

 that the shell is divided into two regions. These are an 

 outer region, formed by the regularly superimposed and 



