228 On Aquatic Carnivorous Coleoptera or VytiscidoB. 



(Silphomorpha) its epimeron is Dytiscidiform in shape, yet the group differs 

 widely from the Dytiscidce 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 Dytiscidse or Carabidae, the epinxeron 

 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 Dytiscidse or Carabidse ; the epimeron is large, but quite different in form 

 from any Dytiscidte, for it has no trace of any superior metathoracic angle, and its 

 greatest 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 Carabidse. 



The Metasternum. 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 in the Macro-Dytiscidse 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 prrescutum, 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 Laccojihilini, 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 



