58 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



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. 71 Salt Lake 



76 94 Salt Lake 



8 1 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 Monophora, 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 



