124 BRITISH OAK GALLS. 
English name of gall.—‘‘ The Oyster Gall.” 
Position of gall.—On the under-surface of the mid-rib and its 
offshoots. 
Manner of growth.—Glabrous, glossy, ovoid, spherical. 
Colours.—Pale yellow, yellowish-green, pink, pale brown. 
Average dimensions of a mature specimen. —. Ovoid: 
Height, 2mm.; length, 3mm.; breadth, 2mm. 
May be sought during the months of August to October. 
Growth is complete by the end of September. 
The typical condition of the gall is unilocular and unilarval. 
The larva pupates in the gall. The imago emerges during 
March. When kept under favourable conditions, imagines will 
appear from galls, which have matured early, during November ; 
but from others late in maturing the imagines do not emerge 
until March. 
Parasites, Nos. 5, 44, 79. Inquilines, Nos. 141, 142. 
Alternate sexual generation: Spathegaster Aprilinus, Gir. 
Leaves bearing from 12 to 30 galls are frequently 
seen. I found during September, 1901, near Hastings, 
a twig of @. pedunculata having eight leaves, all of 
which were galled, the total number of galls upon 
them being 207. The largest leaf measured 115 mm. 
long, and had 42 galls upon it. 
The only galls with which this species can be con- 
fused are those of Dryophanta divisa, and Biorhiza 
renum. It is however more frequently met with than 
the former, and less frequently than the latter. When 
mature it is smaller than D. divisa, and seldom 
spherical. Other distinguishing features are given in 
‘the description of B. renum. 
Growth is rapid, and complete in about four weeks. 
When the gall emerges from the mid-rib or its off- 
shoots it is enveloped by a thin membrane of leaf- 
tissue, which splits longitudinally and forms two flaps, 
or valve-like parts, at the base of the gall; these in 
time wither and fall away. It is in consequence of 
these flaps bearing a fanciful resemblance to the valves 
of an oyster shell, that the gall received its trivial 
name. 
During the earliest stage of development the gall is 
uniformly coloured, but later, numbers of little spots 
