Dr Johnston on the genus C aligns of' Leach. 293 
the abdomen, which none of the others, and of them I had a 
considerable number, possessed. 
Fig. 12. PI. VII. is an outline of what I consider as the female, 
considerably magnified, and Fig. 13. represents the male. Be- 
tween all the parts of the thorax, in both there is a strict agree- 
ment, with the exception of the fourth pair of legs, {uncinuli f 
Muller, ) which in the male are very large, in proportion to the 
others, strongly toothed on the inner margin, and armed with a 
stout claw ; while in the female these legs are small, and without 
teeth. The pectinated membranaceous laminae affixed to the 
inferior angle of the clypeus, are also somewhat larger in the 
former than in the latter. But the chief difference is in the ab- 
domen. In the female, it consists of three very dispropor- 
tionate segments, which, however, are not separated by any dis- 
tinct lines from one another. The first is very small. The se- 
cond is large, narrowed above and truncate behind, where, on 
each side, is attached a cylindrical jointed process, (Jilamenta ovi- 
fara of Muller J, in length exceeding the whole body. The 
third segment might, aptly enough, be termed the caudal. It 
is a very small process, projecting between the filaments, termi- 
nated behind by two papillae, which again have on their extreme 
points three short transparent setae. In the male there are no 
Jilamenta ovi'para. The abdomen is smaller in proportion to 
the thorax, and its segments are more distinctly marked. The 
first is small, angulated, and bears the seventh pair of legs, which 
are in every thing similar to those of the female ; the second is 
broader, considerably larger, rounded before, truncate behind, 
with two small spines at each external angle ; the third segment 
is considerably less than the second, cordate, terminated behind 
by two papillae, whence, as it were, issue from each, three acu- 
minated spines, (of which the mid-one is the longest), finely ci- 
liated an each side, and in length nearly equalling that of the 
abdomen, (Fig. 14.) These particulars are well enough ex- 
pressed in the figures, which will give a more correct idea of 
them, than any description can convey, though they are neither 
so accurate nor so finished as I could have wished. 
Should these observations be verified, they will necessarily 
introduce a material alteration not only into the specific, but ge- 
neric characters. These, however, can be formed by him only 
