IV.—SynTHEsIs, OR CLASSIFICATION. 
FIRST SYNTHESIS (GENERA). 
I. 1.—Genus PELOBIUS. (Vide p. 259.) 
This aggregate is formed by three species; their individuals are of moderate size 
‘(about 10 m.m. in length), the form is short and broad, very convex beneath, the 
surface very densely punctured, without pubescence. 
The head is exserted from, and little clasped by the angles of, the thorax; the 
eyes are convex, prominent, and not touched by the thoracic angles; the labrum is 
quite exposed in front of the clypeus, and perfectly continuous in plane with it; the 
suture separating the clypeus, or epistoma, from the forehead, is angulated in the 
middle (and is very distinct in the European, nearly obliterated in the Australian 
species); the buccal and gular sutures are fine but distinct in Dytiscus tardus, nearly 
obliterated in Pelobius niger; the antenna is inserted in a small cavity situated at 
the upper part of a rather large, subtriangular glabrous space, and is 11-jointed; the 
basal joint is much larger than any of the others and more or less punctate, the 
other joints are without punctuation, but each bears on the upper surface quite at 
the extremity, a minute round cavity; none of the joints has any pubescence, or 
elongate setee; but by very minute examination it can be seen that there are, at any 
rate on the three or four basal joints, the rudiments of a few sete. 
The mentum is very short, its lateral lobes extremely short ; the palpi are rather 
short and stout; the outer lobe of the maxilla is rather obscurely biarticulate. 
The prothorax has the sides of the upper surface distinctly, and the anterior 
mareins both of the pronotum and prosternum densely ciliate ; the prosternum along 
the middle, assumes directly at the front margin a vertical direction, and shortly 
thereafter is turned backwards and continued so as to form a thick, prominent, 
prosternal process; this process is not broad but is elongate, and is nearly acuminate 
at the apex, it is margined at the sides, and the margins are continued forwards 
between the front legs, till they attain the anterior vertical portion of the prosternum; 
the prosternal sutures are indistinct as are also those of the side pieces; the form 
of the prothorax is such as to give it a great extension in the vertical direction; thus 
the depth from the hind margin of the pronotum to the apex of the prosternal process 
384 m.m., this measurement being distinctly greater than the width of the prosternum ; 
this considerable depth enables the front coxee to be conical and elongate; their 
cavities are closed behind by a large corneous process, connecting the inner faces of 
the two epimera. 
TRANS. ROY. DUB. SOC., N.S., VOL U1. 50 
