228 On Aquatic Carnivorous Coleoptera or Dytiscide. 
(Silphomorpha) its epimeron is Dytiscidiform in shape, yet the group differs 
widely from the Dytiscidee inasmuch as the anterior pieces are very large, 
and the epimeron does not penetrate to the coxal cavity. In Pelobius the 
mesothorax might pass as belonging either to Dytiscidee or Carabide, the epimeron 
being rather small and its superior metathoracic angle little developed, so that In 
this respect it is Carabidiform ; while as regards its direction it is indeterminate, 
and with respect to the size of the anterior pieces it is Dytiscid rather than Carabid. 
In the Haliplides too, the mesothorax fails to support their classification either 
with the Dytiscide or Carabidee ; the epimeron is large, but quite different in form 
from any Dytiscidee, for it has no trace of any superior metathoracic angle, and its 
ereatest length (in the true longitudinal direction) is not at the line of this angle, 
but near to the coxal cavity; the anterior pieces are quite small, and different 
therefore from the Carabide. 
The Mzrastrrnum. The metanotum is of much larger area than the mesonotum, 
but in those forms in which the mesothoracic scutellum is least developed (Hydro- 
vatini, Hyphydrini, Laccophilini, and, but not so markedly, Hydroporini) it is much 
smaller than in those having the scutellum largely developed and of very firm con- 
sistency, as it is inthe Macro-Dytiscide generally. It is of complex structure, and 
it appears to me very difficult to determine accurately of how many pieces it is 
essentially composed, and to decide as to their homologies. It is of course covered 
entirely by the elytra, and although it is horny, it is thinner than are the exposed 
parts of the skeleton. It is traversed in the middle, by a deep and broad longi- 
tudinal depression, into which is received an incrassation on the basal portion of the 
inner face of the suture of the elytra. In the middle in front it is armed with a 
small angular vertical projection, which acts to some extent as a support to the free 
extremity of the mesothoracic scutellum, and which probably represents a portion 
of the preescutum, which is otherwise absent: this projection is not present when 
the scutellum is rudimentary (Hyphydrini, Hydrovatini, Laccophilini), and is small 
in the Noterides, and Hydroporini; in fact its development appears to be strictly 
correlative with the growth backwards of the scutellum. The larger part of the 
metanotum is occupied by the scutum, which is divided in a more or less irregular 
manner, longitudinally and transversely so as to form four pieces. The anterior 
pieces—one on each side—are shining and glabrous, and are especially reduced in 
size in the Hyphydrini, Hydrovatini and Laccophilini, while in the Noterides these 
plates are small, and placed chiefly in the vertical instead of in the horizontal 
direction. Behind these two anterior plates there are placed the two posterior 
plates of the scutum ; these are not shining like the anterior ones, but are dull and 
frequently bear a few hairs. Behind the scutum, the scutellum is visible as a 
transverse linear band on each side, but in the middle it has a considerable extension 
in the anterior direction, by encroaching on the hind border of the posterior plates 
of the scutum. The post scutellum is placed behind the scutellum, and on a rather 
