142 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
Cladocera, it would seem that the evidence is conclusive that the latter - 
group is not the direct continuation of the line of development inau- 
gurated by an ostracode ancestor. As shown beyond, the present 
centre of the group seems near Moina, with indications of a divergence 
from this rather generalized type, especially of degradation and _ het- 
eronomy on the side of the Lynceids. 
It seems at the present time that more might be accomplished for 
etiology by a careful study of such groups as the present, in which 
are a variety of closely allied forms, than by the attempt to join 
widely separated groups. When we shall have seized upon the latest 
eddies and mapped their direction, it may become possible to combine 
the indications in such a way that lines of divergence thus traced 
accurately through some small part of their course may be produced 
backward to their intersection. This, then, is our present duty—the 
accurate mapping of minute districts and the careful noting of any 
moving straws competent to indicate movements in the vast complex 
of vitalized nature. We conceive the Cladocera to have had a com- 
paratively recent origin, and to express the culmination and retrogade 
development of a plan of structure first differentiated after the appear- 
ance of clear bodies of fresh water. All the species save a very few 
are confined to inland waters. Accepting the above mentioned theory, 
the Sidide will occupy the first place as departing least from the type 
from which the whole group sprang, while it is connected by the genus 
Daphnella with the Daphnide. The Daphnide, beginning with Moina, 
find their ultimate development in some monstrous forms of the genus 
Daphnia, but pass into the Lyncodaphnide by way of Macrothrix. The 
links uniting all these minor groups are very obvious. 
Our ownideas of the relationships among the Calyptomerous Cladocera 
are expressed in the accompanying table. This table is to be consid- 
ered a projection of a portion of a genealogical tree, seen from below, 
in which the genus Moina forms the arbitrarily chosen fixed point. 
The heavy dotted line is imagined as directed downward vertically. 
That branch rising toward the top of the page is growing obliquely 
upward. The Daphnide are represented as expanding upon the same 
plane as Moina, and the Lyncodaphnide extend diagonally downward, 
producing the Lynceid branch. The Bosminide spring from the stem 
at a lower point.* 
The Cladocera or Daphnoidea are characterized by the more or less 
leaf-like feet and the lamina of thin chitine which incloses the greater 
* Nore.—To adapt the diagram to the theory that the Lynceide@ are the progenitors of Cladocera, it is 
only necessary to revolve the imaginary line to the right, till it coincides with the axis of that family. 
The question mark may be understood to indicate that the source of the pivotal group, Moina, is uncer- 
tain. The author must confess that his inclination is toward a belief that the line culminating in the 
Daphnide diverged from a group of organisms resembling Phyllopoda, more definitely, resembling 
Limnetes. There is a very remarkable resemblance between the larva of Limnetes and Bosmina. The 
lateral spines of the former are, as will be shown, true homologues of the antennules of Bosmina. The 
later origin of the Phyllopoda io their present form may be well admitted. 
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