36 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
tween these two species? Though the females of violaceus, densareolatus, and 
platycephalus are so similar, the males are nevertheless very different in regard 
to the form of their posterior ends, so that these species may be regarded as 
genetically related, and we must consider that the course of modification which 
they have undergone has influenced the males more than the females. This 
view of the question cannot be regarded as bizarre, since in other groups of 
animals also the males are in some cases far more dissimilar than the females. 
Especial Diagnostic Characters. The flattening of the anterior end, and the 
constriction of the head from the body ; the slight enlargement of the posterior 
end in the female; the absence of spicules on the tail lobes in the male; the 
presence of small, rounded-polygonal cuticular areoles, which are as a rule well 
separated from one another. 
Geographical Distribution. Guatemala, South Montana, Pennsylvania, 
Bridger Basin, Fort Laramie. 1 male and 7 females examined. 
6. G. platyurus Baird. 
Figs. 50-52, Plate 7. 
G. platyura Baird, ’53. 
G. platyurus Baird, Diesing, ’61. 
G. platyurus Baird, Villot, ’74. 
G. platyurus Baird, Oerley, ’81. 
G. platyurus Baird, Romer, ’96. 
(1 female examined: Leidy coll. 5096.) 
Form. Very massive, flattened dorso-ventrally, with broad dorsal and ven- 
tral grooves, which do not extend quite to the posterior end. Head end (Fig. 
52) conical. Largest diameter posteriorly. The posterior end (Fig. 51) is 
dorso-ventrally flattened, expanded, wider than the preceding portion of the 
body, spatulate in form, with shallow dorsal and ventral depressions. The 
cloacal aperture is terminal. 
Cuticle (Fig. 50). With fine intersecting lines, much as in G. aquaticus 
Linn. Here and there bundles of elevated lines are demarcated from the finer 
lines, and these bundles, which are parallel to the finer lines, deliminate 
rhomboid-shaped spaces. 
Color. A light yellowish buff, somewhat iridescent. Extreme tip of head 
white, behind which is a faint brownish ring. Posterior tip of the body a light 
yellowish white. 
Dimensions. Length, 540 mm.; greatest diameter of body, 2.4 mm. ; great- 
est transverse diameter of tail, 2.3 mm. 
No locality is marked for this specimen ; the only other specimen known, 
the type in the British Museum, is labelled “ Jamaica?” so that it is not yet 
proved that this species is American. It seems to me probable that it does not 
come from the North American continent, since otherwise there would proba- 
bly be numerous examples of this large species extant. Its massive form and 
the spatulate shape of the posterior end are good diagnostic characters. 
