824 [Assembly 
species has a dusky spot below the knees which does not appear in ours. It also has 
a double row of whitish lines, more or less distinct, towards the outer margin, which 
in our species is replaced by a single row of whitish spots. Other differences might 
be specified, but these suffice to show the glaber distinct from its European 
analogue. 
The Unspotted Porcellio (P. immaculatus) is dull blackish brown or leaden 
brown with faint short pale lines and the middle of each segment rough from elevated 
granules; under side and legs white or lurid. Length 0.30 or less. This is readily 
discriminated by its uniform brown color unvaried by spots or stripes save the short 
longitudinal lines which are so faint as scarcely to be perceived and are frequently 
wholly wanting. It is also our smallest species. It probably occurs throughout the 
United States, for I met with it in Illinois, and specimens have also been sent me by 
Mr. Robertson from west of Arkansas. 
The Striped Porcellio (P. viltatus) is black or leaden blackish with the head 
deeper black and the under side whitish; the segments are rough from elevated gra¬ 
nnies with their hind margins smooth; along the middle of the back is a row of white 
spots and another more distinct near the outer margin; these spots are often conflu¬ 
ent, forming continuous stripes. Length 0.35. The same pale short longitudinal 
lines which are common in other species are more or less perceptible in this also. 
Young individuals are of a pale or even whitish color but show the usual stripes of a 
more clear white. It is one of our most common species. 
The Mottled Porcellio (P. mixtus) is tawny yellow variously dotted and spotted 
with black, and with a row of whitish spots which are often confluent into stripes 
along (he middle of the back and near the outer margin; outer edge pale, at least on 
the angles of the segments; segments rough from elevated black granules, their basal 
and apical margins smooth. Length 0.40. The elevated granules form round and 
oblong black dots, and often on each side of the back the intervals between them are 
white, thus presenting short longitudinal lines of this color, and in a variety ( varie■ 
gatin') these lines are confluent, forming a longitudinal row of white blotches be¬ 
tween the dorsal and lateral stripes. Sometimes the stripe on the middle of the back 
is tawny yellow instead of whitish. This appears to be the most rare of any of our 
species. 
The Pretty Porcellio (P. limatus). Black or blackish, with a stripe each side 
and the outer margin broadly whitish, and two rows of bright yellow spots along the 
back; the segments rough with raised granules over their whole surface. Length 
0.60. This is our most common species, being thrice as numerous as any other. It 
occurs in abundance in our cellars and under stones and billets of wood in the yards 
about dwellings and barns. It is quite variable in its colors. In young individuals 
the two rows of spots along the back are pale or whitish. As it increases in size they 
all gradually change to yellow, or one or two of these spots take on a bright yellow 
color whilst the rest remain whitish, but this yellow color is successively assumed by 
the others, and in old individuals the whole become of a vivid ochre yellow. Dots 
of this same color sometimes appear also upon the narrow posterior or caudal seg¬ 
ments prolonging the rows to the tip of the body. The following varieties of this 
species may be distinguished. 
