334 Merrill—Structure and Affinities of Bunops Scutifrons. 
seems to be some affinity between this species and Bosminaff but 
Bunops shows no such affinity. If Herrick’s imperfectly de¬ 
scribed form belongs to this genus, it is very different from 
the species under consideration. 
RELATIONS OF BUNOPS IN THE FAMILY MACROTHRICID JE. 
The family Macrothricidm is small, distinguished by many 
genera and few species, and the genera show many interesting 
cross affinities. There are nine genera, each containing one to 
five species; Macrothrix , five; Drepanothrix ) one; Lcitho?iura y 
one; Ophryoxus , one or two; Acantholeberis , one; Grimaldina , * 
one; Guernella , * one; Ilyocryptus , three European species and 
two from the southern hemisphere; Bunops , two. In marked 
contrast with this family with its large number of genera, each 
containing very few species, is that of the Daphnidce , contain¬ 
ing six or seven genera, of which the genus Daphnia alone com¬ 
prises some forty or fifty described species. The family Lyn- 
ceidce has some twelve genera, and the genus Alona contains as 
many as thirty species; Chydorus nearly twenty species; with 
other genera equally as large. Both the. Daphnidce and Lyn- 
ceidm thus have shown a capacity for specific variation, far 
greater than the Macrothricidce. The genus Bosmina ) the sole 
representative of the family Bosminiclce, has at least twenty-five 
American and European species, a greater number than the nine 
genera of the Macrothricidce. But while the Macrothricidce are 
deficient in number of species, yet in genera, and in range of 
form and structure, they compare favorably with any of the 
larger families. 
There is a curious parallelism of development in all of these 
three families, Daphnidce , Macrothricidce and Lynceidm. In 
each, we find genera whose form is approximately spherical, and 
these genera are the smallest in size in the family. Ceri- 
odciphnia among the Daphnidce ., Strehlocerus in the Mac- 
rothricidce , Chydorus and Ancliistropus in the Lynceidm , are re¬ 
spectively the smallest genera of those families; while the ro¬ 
tund Pleuroxus nanus is the smallest of the Cladocera. In each 
* African genera described by J. Richard. 
