84 Trans. Acad. Sci. of St. Louis 
posterior end. In an effort to locate thicker wood an 
individual may occasionally withdraw its shell and the 
anterior portion of its body from the anterior portion 
of the tube, forming a new tube in a different direction. 
At the point of separation from the original tube a 
diaphragm is formed completely sealing the abandoned 
tube. This operation may be repeated several times by 
a single individual (pl. 5, fig. 3). Tubes of normal speci- 
mens of the same species which are 400 millimeters in 
length are approximately twelve millimeters in diameter 
at the anterior end. This abnormal form which occurs 
in thin wood, however, possesses none of the characters 
of the stenomorph. The thickness of the shell, arrange- 
ment of the denticulate ridges and the proportionate 
sizes of the anterior and anterior-median areas, being 
the same as in normal specimens, the abnormality 
caused by the thickness of the wood can be seen only in 
the excessively narrow form of the tube. The large 
proportion of abnormal specimens with attenuate tubes 
and the small number of true stenomorphs found in the 
shingles, would tend to show that the stenomorph 
characters are not caused by the size of the wood. 
Also, many of the small test-blocks used in 1922 con- 
tained in one portion of the block many hundreds of 
stenomorphic specimens, while in another equally large 
portion of the block there were only relatively a few 
normal specimens of the same species. For example, 
small test-blocks from Galveston, Texas, contained for 
several weeks during the summer of 1922, in one-half 
of the block, hundreds of specimens of stenomorphic 
Bankia gouldi, few being more than twenty millimeters 
in length, whereas in the other half of the block there 
were always two, three, or more large normal 200-300 
