66 Le Conte on the Pselaphide of the United States. 
axis are provided with only ten joints, when the medial thora- 
cic fovea is larger than the lateral ones, or when these last are 
entirely wanting. 1 have not yet succeeded in finding which 
joint becomes obsolete, but it would be very interesting to fol- - 
low this up by the aid of foreign species, and to ascertain 
whether other forms than those specified are also embraced in 
the division with ten-jointed antennas. "The genus Batrisus 
having been very much increased, it became necessary to 
search for some means of dividing it into groups. The struc- 
ture of the front has enabled me to do this in a tolerably nat- 
ural manner, and thus to point out more clearly the specific 
characters belonging to each; and the peculiar sexual varia- 
tions established under each group, will be sufficient proof 
that I have not unnecessarily multiplied the species of this 
heretofore limited genus. 'The characters used by me in 
dividing this genus having unfortunately been passed over in 
a negligent manner by former writers, I am still uncertain 
about the position of the foreign species, and of the relation 
which they bear to ours: I have, however, for the sake of 
uniformity endeavored to intercalate them as well as their 
descriptions would permit me. 
The parallelism of the productions of the two continents is 
seldom more beautifully displayed than in the present group- 
There is now but a single European genus (Bythinus,) which 
has not its representative or its analogue in this country. AS 
will be seen by the following table, this correspondence 
extends in a remarkable manner through the species of which 
the genera are composed, so that almost every species of tbe 
genera common to the two continents has its exact equivalent: 
moreover in those genera, which, from possessing less powe” 
of variation, eonsist only of single species, the parallel- 
ism of characters is still kept up to a most surprising extent; 
so much so, that if we were to leave out all strictly generic 
characters, the two analogues would be represented by 3 
single diagnosis. 
In addition to these equivalents and analogues we have 
still remaining many genera and species without representa- 
