250 MR. E. 8S. RUSSELL ON THE SHELL-GROWTH | Mar. 2, 
5 < oo 1h : 
accounts for the decrease of the ratio ul and the inerease of the 
: : oa AUD Is 
ratiop. The change in the ratio = is chiefly due to the same 
> 
cause, and especially to the fact, easily observed, that the 
“inerowth” at the posterior margin is much greater in extent 
than it is at the anterior margin, whence it arises that Af becomes 
L 
larger in proportion to Ab. In the three cases in which a 
increases and q decreases during growth from one group to the 
next, one must suppose that an “ outgrowth” has taken place, 
caused probably by the formation of very large projecting ribs, or 
else that the apex of the shell has been ab ‘raded. The latter 
explanation is probably the true one for the 55-60 mm. shells, 
for such old shells are usually very much worn and eroded, 
especially towards the apex. 
The change in the ratio a may possibly be due also in part 
to the unequal rates of growth of the anterior and posterior 
margins. There is some reason to think that the anterior margin 
grows a little quicker than the posterior. If one examine a 
well-preserved “smooth” shell (see 9. p. 868) from above, one 
sees numerous fine ribs radiating from the apex. Now it very 
often happens that many of the lateral ribs curve slightly 
forward, while the anterior and posterior ribs run out in a 
straight line from the central apex. This shows that during 
growth the mantle (or the whole limpet perhaps) gets shitted 
forward a little relatively to the apex, and that accordingly the 
growth of the anterior margin becomes somewhat greater im 
proportion to that of the posterior margin. The ribs of the shell 
end in little projections of the rim, and to these projections of 
the rim correspond little projections of the edge of the mantle, 
hence from a curving forward of the ribs one can infer a 
movement forward of the mantle relatively to the shell-apex. 
Tn addition to the radiating ribs there are visible on unworn 
shells numerous lines of growth. These are often very distinct, 
especially at about the level corresponding to lengths of 15 mm. 
and 30 mm. The shell lying between these two levels is probably 
added during the first summer and autumn’s growth. It is, 
however, hardly possible to recognise annual lines of growth on 
the shell other than these two. 
Shells of 40 mm. and more are usually very much worn on 
the upper part, from a level of 30 mm. upwards. 
Besides the changes in shape during growth there is also a 
marked change in the build of the shell. It becomes very much 
thicker and heavier. This increase in thickness (out of proportion 
to the increase in length) becomes noticeable from a length of 
about 25-30 mm. onwards, 7. e. in one year old shells and older 
ones. The rim becomes considerably thickened, and a great deal 
of shell-material is laid down inside the apex, which soon becomes 
solid to quite a considerable depth. In a limpet of 60 mm. the 
thickness of the shell at the apex was so great as 10 mm. 
