320 Merrill—Structure and Affinities of Bunops Scutifrons. 
where the dorsal joins the ventral edge of the valves. The gen¬ 
eral semicircular outline of the ventral margin is broken by a 
slight concavity at its most ventral part caused by the curling 
or infolding of the ventral edges of the valves. Viewed from 
below, (PI. XIV, Figs. 2, 6), the posterior half of the ventral 
edge of the valves is seen to be sharply inflected to form a right 
or acute angle with the main part of the valves, causing the 
valves to gape widely at the anterior part, even when the pos¬ 
terior parts are tightly closed. The anterior ventral margin is 
edged by long, smooth, straggling, movable hairs. Just in front 
of the infolding is a space where they are short and feeble. 
Along the infolded part they are short and densely set, and 
cease altogether a short distance from the posterior end. At 
the anterior part of the ventral edge of the shell, the hairs are 
placed alternately in and out, but upon the bend, out only. On 
the infolded part they point inward. The infolding of the ven¬ 
tral edge sets the posterior hairs up a little, so that there is a 
space between them and the apparent edge of the valves as 
seen from the side. The valves are reticulated with hexagonal 
meshes in the anterior portion. Similar reticulations are seen 
on the head. The keel and posterior part of the valves are 
smooth. 
HEAD. 
PI. XIV, Figs. 1, 4. 
The head is small, triangular in general outline, and is very 
short antero-posteriorly. It is shorter and smaller in propor¬ 
tion to the size of the animal than in any other genus of the 
family. It is not depressed and the rostrum projects forward. 
The crest extends over the head about one-third of its length 
and then widens out into a triangular area, which is continued 
as a flattened rectangle to the end of the rostrum. This area 
is thus somewhat kite-shape or shield-like in outline. Its edge 
is formed by a strong ridge, whose lower part probably repre¬ 
sents the continuation of the fornices. The general surface of 
this area is depressed and the sides of the head are compressed 
just behind the boundary line. This area I call the forehead. The 
even surface of the forehead is broken by a very pronounced, al- 
