]42 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 Mohia, with indications of a divergence 

 from this rather generalized type, especially of degradation and het- 

 eronomy on the side of the Lyncekls. 



It seems at the present time that more might be accomplished for 

 setiology 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 Sididw will occupy the first place as departing least from the type 

 from which the whole group sprang, while it is connected by the genus 

 DapJmella with the Daphnidw. The Daphnidm, beginning with Moitia, 

 find their ultimate development in some monstrous forms of the genus 

 DajyJmia, but pass into the Lyneodaphnidm by way of 2Iacrothrix. The 

 links uniting all these minor groups are very obvious. 



Our own ideas of the relationships among the Calnpiomerons 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 growling obliquely 

 upward. The Dajihnidw are represented as expanding upon the same 

 plane as Moina, and the Lyncodaphnidw extend diagonally downward, 

 producing the Lynceid branch. The Bosminidw 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 



* Note. — To adapt the diagram to the theory that the Lynceidm are the progenitors of Cladocera, it is 

 only necessary to revolve the imaginary line to the right, till it coincides with the axis of thai 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 

 Daphnidoe diverged from a group of organisms resembling Phytlopoda, 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 oi Bosmina. The 

 later origin of the Phyllopoda in their present form may be well admitted. 



