62 
PHYLLIVORA. 
tral one in form, but also from each other as they 
approach the edge of the membrane. The general 
form is subquadrilateral, the anterior and posterior 
edges being subject to the same variations as those 
described with reference to the central plate, while 
the inner edge is always more or less convex, and 
the outer edge concave. In those species where the 
curve of the horizontal row is considerable, the 
plates as they approach the edge get narrower, and 
in these it is not unusual for them also to assume 
somewhat an S form on the one side, and its reverse 
on the other. In others, however, the lateral plates 
become gradually broader, and eventually twice as 
broad as the primary lateral plates. 
The lateral tubercles vary nearly as much in the 
same individual as they do in different species. 
If an ideal line be drawn longitudinally through 
the central tubercle, so as to divide it equally, it 
will be found that the two halves are precisely 
similar (fig. 20.), — but such is not the case with 
any of the lateral tubercles, those lateral tubercles 
which are nearest to the central tubercles are always 
more similar to it in general form than those at the 
edges, — indeed, that the tubercles become more un- 
like the central tubercle as their position is nearer to 
the edge of the membrane — the lateral tubercles, 
indeed, are merely modifications of the forms of the | 
central tubercle,— and that these modifications are 
effected by the suppression of the prominence on the 
inner side of each lateral tubercle, and the simulta- j 
neous increase of the corresponding part of the outer 
side. 
