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 tviberclc, and the simulta- 

 neous increase of the corresponding part of the imier 

 side. 



