On Aquatic Carnworous Coleoptera or Dytiscida. 211 
times so as to form two or more fine pencils, but sometimes extending along the whole 
margin in an equal manner: the outer surface of the ligula is impressed on each 
side for the accommodation of the labial palpi, so that along the middle it is more or 
less bulged or prominent: on the inner face the ligula is membranous, except that 
there is a transverse horny strip at the extremity. The paraglossz are closely united 
to the ligula, and do not project beyond it ; on the outer face of the ligula they are 
difficult to detect, but are nevertheless visible on each side, as forming a more 
transparent corneous outer edge to the ligula; but on the inner face of the ligula 
they are very conspicuous, as each of them has its inner margin, fringed with an 
elongate series of very fine, dense ciliz. The supports of the labial palpi are 
conspicuous and at their base are attached to the outer face of the ligula, and covered 
by the front portion of the central piece of the mentum ; their extremity is on the 
contrary very conspicuous, and appears in the form as it were of a stout basal joint 
to each palpus ; this joint bears a deep notch on its antero-external face and thus the 
palpus is capable of being bent back over the mentum as well as moved in a forward 
and dependent manner. The labial palpi are of moderate length and are three- 
jointed: the basal joint is considerably shorter than either of the others, while the 
second and third differ but little from one another in length (except in the Noterides) ; 
the second joint frequently bears a minute prominence on the middle of its inner face 
causing it to appear faintly bi-emarginate on this face: the apical joint is usually 
simple and cylindrical, but may be dilated, and bear a notch (Coptotomus). 
In the Noterides the labial palpi are peculiar, the basal joints being short, 
while the apical one is large, more or less dilated, and bears a notch: in 
Hydrocanthus this dilatation of the terminal joint of the labial palpus becomes 
extreme. 
The labial palpi and the outer face of the ligula, are, like the maxillary palpi, polished 
and shining and destitute of any exserted sete. In Pelobius the ligula shows much 
difference from that of the Dytiscidz, it is short, and its anterior portion is flexed 
upwards so as to assume an obliquely vertical direction, this semi-vertical portion is 
rather coarsely punctate. I can detect no paraglosse, and the supports of the labial 
palpi are not emarginate. In Amphizoa the ligula does not project beyond the 
lateral wings of the mentum, owing to the great development of these latter, but 
its structure is quite that of the Dytiscide. 
The parts of the mouth in the Dytiscide offer two points of interest—first, the 
great uniformity they show throughout the family, and second, the way in which the 
different pieces are coadapted so as to close together quite completely and exclude 
the entry of water : the mandibles are curved so that they close under the labrum, 
and of the maxillee only the very hard, hooked, extremities are at all exposed, the 
ligula and mentum completely closing the underside of the mouth ; notwithstanding 
this close packing together of the parts of the mouth the whole of the maxillary 
and labial palpi are entirely exposed—more completely, in fact, than is usual. 
TRANS. ROY. DUB. SOC., N.S., VOL. Il. 2F 
