IDOTEA, 377 
the tip. At the base of each of the foot-jaws is attached 
a large movable sub-oval plate. The mandibles are 
robust, with a strong, curved, denticulated apical blade, 
below which is a small movable plate, and a_ broad 
truncated molar tubercle: they are destitute of an arti- 
culated palpiform appendage. 
The outer maxillz are very delicate, and terminated 
by three oblong strongly ciliated plates ; the inner max- 
illa have an elongated base and two thin terminal lobes 
of unequal size, ciliated at the tips: the lower lip is 
thin, with a deep incision in the middle of its anterior 
margin (fig. * in page 379). 
The legs are all formed for walking, rather than for 
prehension or swimming; they are nearly of uniform 
structure, the anterior pair being not larger than the 
following ; they, however, increase gradually in length to 
the hind pair. In the female, the third and three follow- 
ing pairs of legs bear at the base of each a large mem- 
branous plate, folded beneath the body, forming a large 
ovigerous sac. ‘The tail consists of a very large terminal 
flattened shield, preceded by three very short segments, of 
which the third is dorsally fused with the next succeeding. 
In I. Entomon, a Baltic species, there are three distinct and 
two apparent joints in this part of the body; in other 
species, the soldering together more or less completely of 
two or more of the short basal joints takes place, until in 
I. appendiculata the tail appears to consist of only a single 
large plate-like shield, traces of the articulations being 
only visible at the lateral margins of the basal portion of 
the joint.* The normal six-jointed structure of the tail 
is, on the contrary, proved on the ventral surface of the 
* The variations in the condition of these basal segments have furnished 
Professor Milne Edwards with the characters for distributing the species into 
various minor groups. 
