58 
PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
to the “alate” form, and of these only the six largest (over 55 mm long) 
show this character distinctly developed. In the young ones the alate 
form is brought about chiefly by a comparative narrowing of the anterior 
end, not by an increase of the width as compared with the longitudinal 
diameter, although such forms are always a few millimeters broader than 
long. Thus, in very young individuals, the alate and rotundate forms 
are not so very strikingly different from each other, and, indeed, in some 
cases it is hard to say whether a particular individual should be classed 
with the one or the other. With increasing age this difference becomes 
more striking, and at an average length of about 50-55 mm both forms 
may be easily distinguished at a glance. The form “rotundatus” in 
young specimens is distinctly pentagonal, and also the young of “alatus” 
are irregularly pentagonal. But the pentagonal shape disappears with 
age, becoming sometimes “cordiform,” when there is a posterior median 
emargination. 
The rotundate form never attains the size of the alate. Lahille’s largest 
rotundatus is 65 mm long, 67 mm broad. We have a fragment, appar- 
ently belonging to the rotundate form, that has a diameter of 70 mm, 
while the largest complete individual is only 62 mm in diameter. Some 
of our alate specimens are very much larger than Lahille’s (his largest is 
66 by 72). I give here the measurements of our six largest individuals: 
Long Diameter. 
Transverse Diameter. 
Locality. 
56 
68 
Lake Pueyrredon 
59 
65 
Shell Gap 
62 
75 
Salt Lake 
64 
ca. 7 1 
Salt Lake 
76 
94 
Salt Lake 
81 
89 
Lake Pueyrredon 
These measurements show that the relation between length and width 
is extremely variable, some of our specimens being much more alate than 
any of Lahille’s. 
As to the meaning of the existence of two such forms (rotundatus and 
alatus) within this species, I can only refer to Lahille’s opinion : he com- 
pares this fact with the analogous case in Monop horn, in which he believes 
(1896, p. 10), that these forms represent the female (rotundatus) and the 
male (alatus) of the same species. I cannot offer any further evidence 
for this theory, with only the exception, that the fact that the alate or 
