500 MEMOIRS OF THE CARNEGIE MUSEUM 
C. bartoni, differs from the rest ecologically as well as morphologically. It is dis- 
tinguished by a number of characters, and there is no possibility of morphologically 
intermediate forms, so that C. bartoni not only is a good species, but also belongs to 
a different section of the subgenus. 
C. bartoni possesses in Pennsylvania a variety, C. bartoni robustus, which, accord- 
ing to my experience, is constant, and never runs into the typical form. It also 
seems to occupy a slightly different territory, although often found associated with 
the latter. ‘These facts would justify us in regarding it as a good species. I have 
not done so in the systematic part, since the facts at hand are too meagre to finally 
decide this question. The range of C. robustus in Pennsylvania is only a small part 
of the area occupied by this form, and in the states of Ohio, New York, and in 
Canada, the conditions are entirely unknown. Furthermore a form similar to our 
robustus, although, as it seems to me, not entirely agreeing with it, has been reported 
from Virginia, Maryland, and Kentucky, and before particulars about the relation 
of this form to C. barton and to our robustus are known, we cannot judge as to the 
taxonomic position of C. robustus. Therefore I have refrained from modifying the 
position hitherto assumed, that this form is a variety of C. bartoni. 
The other species of the subgenus Bartonius in Pennsylvania are C. carolinus, C. 
monongalensis, and C. diogenes. ‘They belong to the diogenes-section, and all three 
are closely allied. C. carolinus and monongalensis are more nearly related to one 
another than to C. diogenes. ‘The latter apparently is a more advanced form. 
C. carolinus and C. monongalensis are distinguished by rather insignificant mor- 
phological characters, discovered in the shape of the rostrum and the armature of 
the chelipeds. But the difference in color is so striking that it is impossible to con- 
found them in the field. Other characters also, although slight, hold good accord- 
ing to my experience, and I never have seen intermediate specimens. Moreover 
the distribution of these two forms is very characteristic, they being sharply sep- 
arated topographically, and never being found associated at the same locality. Thus 
all requirements leading us to pronounce them good species are met. Of course this 
applies only to conditions in Pennsylvania, Maryland, and northern West Virginia ; 
whether they are the same or different farther south remains to be seen. 
C. diogenes is more sharply separated from the species just discussed, and there 
is no possibility of mistaking this species, more particularly as the color is markedly 
different. But the morphological characters are also very nicely expressed, so that 
in a case of a red (albinistic) specimen of this species I was not a moment in doubt 
that I had to deal with C. diogenes, and not with C. carolinus, although the latter 
was found associated with this form at this particular locality (Dunbar). There is 
