392 MEMOIRS OF THE CARNEGIE MUSEUM 
gives a clue. This, however, is not the case in very young specimens, (less than 20 
mm. long), and such I am unable to distinguish from the typical form. 
The characters are slightly variable, as has been pointed out above, but this 
variety generally is very uniform in its characters in Pennsylvania. I have not 
found any variations worthy of special mention. With reference to the lateral spine 
of the carapace, there are specimens which show no trace of it, (young as well as old). 
In old specimens this spine is often tuberculiform, and in about half of the number 
at hand there is on each side a sharp, but always small lateral spine. In this respect 
there is no difference in the specimens of northwestern Pennsylvania from those 
found in AHegheny County. 
' It seems to me that the southern records for this variety (Maryland, Virginia, 
and also Kentucky)”, do not refer to exactly the same form which is found 
in the north (Canada, New York, northwestern Pennsylvania, northern Ohio). 
Hay (1899, p. 966), in the key to the species gives as one of the differential 
characters of C. bartoni robustus: “carapace cylindrical, sides nearly parallel as 
far forward as cervical groove, then curving abruptly to the base of rostrum,” 
while, under C. barton, the carapace is described as “ . . . depressed, sides gently 
curving toward the front and rear.” his cylindrical shape of the carapace is deci- 
dedly not present in our northern form ; on the contrary, the depression of the cara- 
pace in our robustus is, if anything, more pronounced than in the typical barton ; 
and our robustus agrees in this respect with Girard’s type from Canada, preserved in 
the Academy of Natural Sciences, Philadelphia, and which has been examined by 
the writer. 
On the other hand, our specimens from Kentucky seem to approach the form 
from Virginia and Maryland. ‘he shape of the carapace is more cylindrical, as 
Hay describes it, G: H:B = 1:1.05 to 1.2:1.2 to 1.38. This shows that the width 
of the carapace at the branchial and hepatic regions is decidedly less, compared with 
the vertical diameter at the gastric region, than in the typical bartoni. There are 
other slight differences in the form from Kentucky : (1) the rostrum is not quite so 
narrow ; (2) the lateral spine of the carapace is absent ; (3) the punctures of the areola 
are not so crowded (about three rows), and are similar to those of bartoni; (4) the 
impressions of the hand are indistinct ; (5) the double row of tubercles on the inner 
margin of the hand is different, the outer row being distinct, but the inner consist- 
ing of only a few more or less distinct irregular tubercles. All four specimens from 
Kentucky are comparatively small, (the largest is 54 mm. long), and thus the two 
last described characters may be due to age, although the specimens differ slightly 
Faxon, 1890; Hay, 1899 ; Williamson, 1905 ; Ortmann, 1905b. 
