1915.] 115 



Oil the upperside of the. tiiorax there exists some remarkable 

 channels, one of the most characteristic features of Helophormi, which 

 are very useful in the discrimination of the species. In some species 

 seven of these channels are easily distinguished (Trichelophorus 

 aUerna7is, e.g.). I propose to call these channels " grooves," and the 

 areas between them " intervals." The median groove is straight, but 

 the juxta-inediau one is nearly always made irregular by the dilatation 

 in its middle of tlie median interval ; this dilatation is usually angular, 

 but in if. laticolUs and some others the angular prominence of this 

 interval is absent, or nearly so. The area between the juxta-median 

 groove, and the sub-external groove (i.e., the groove next outside the 

 juxta-median) may be called the sub-median interval, and it is more or 

 less disturbed, or interfered with, by the angularity of the sub-median 

 groove, from which there results, in the extreme case of J^mpleurus 

 nujosiis, the division of the interval into two elevations. 



The sub-median groove is in its general direction parallel with the 

 median one, but the sub-external groove tends to converge basally 

 towards the median groove, in conformity with the lateral outline of 

 the prothorax. The most remarkable feature of the sub-external 

 groove is the presence in it of a small deep pit near the base. This pit 

 is indistinct in the Empleuri where, indeed, it can only be seen when 

 the surface is quite clean ; it is, perhaps, best displayed in Meghelo- 

 phorus. The sub-external interval is usually less definite than those 

 nearer the middle, and the sub-external groove is consequently less 

 definite than the median and juxta-median grooves. The external 

 groove consists merely of the space between the sub-external interval 

 and the more or less raised lateral margin of the thorax, and con- 

 sequently varies in accordance with the modifications in shape of the 

 sides thereof. The sculpture at the bottom of the grooves is always 

 different from that of the intervals, being much finer, or even altogether 

 obliterated, and the metallic coloration of the grooves is usually different 

 from that of the intervals. 



On the under surface of the prothorax there is a modification of 

 structure of an interesting character, there being at the front angles a 

 hollowed space, adapted to receive the back of the eyes when the head 

 is reti'acted. The vicinity of this .ocular cavity is furnished in the 

 Helophori with more or less numerous flexile setae (well exemplified in 

 MeghnlnpJw7'ris aquatic as). These setae exhibit various modifications, 

 and in the Empleuri are quite absent. I briefly described the structure 

 of the elytra, so far as the epipleuron is concerned, on })age 4, and need 



I 2 



