ENTOMOSTRACA OF MINNESOTA. 219 
O. gracilis not only has the dise-like last foot colored but the swim- 
ming antenne are banded with purple as in Simocephalus rostratus 
Herrick and S. americanus Birge. The intestine has anterior cca, 
which is not the case in Lynceids, nor, indeed, in other Lyncodaphnide. 
The four-jointed ramus of the antennze approaches Lynceide in the 
absence of a lateral seta, but the other ramus is as in Macrothrix. The 
convolution of the intestine, the form of the post-abdomen and the 
situation of the anus are all of a strictly Iynceid type; moreover, the 
flattened appendage of the last foot is like that of Hurycercus. 
Even in the form of the shell there is a combination of characters; 
the anterior part of the shell has the form peculiar to Lyncodaphnide; 
but posteriorly it again expands and becomes truncate behind; the 
form in the adult is not unlike that of some Lynceide, but the young 
has a long spine posteriorly exactly like the spine of Daphnia. The 
latter fact is very instructive, for it indicates that the theory proposed 
(American Naturalist, 1882, p. 815) to explain the origin of this ap- 
pendage is probably the correct one. Professor Leuckart suggested 
that this spine was a balancing rod intended to keep the proper equi- 
poise over the center of gravity; but it is difficult to see why these 
long-bodied forms, in which the greater part of the weight lies 
“‘abaft’’ of the pivotal point—the base of the antenne—should be 
thus provided while the shorter forms are not. We conceive that it 
is an apparatus for effecting the moult of the inner lining of the brood 
cavity of long-bodied and tender-shelled animals such as Daphnia and 
the present genus. The great development of the head in the crested 
Daphnide may undoubtedly be explained upon Professor Leuckart’s 
theory. 
* Ofryoxus gracilis Sars. 
PLATES XLII, Fiaes. 12-15; LVIII, Fras. 1-3; LIX, Fia. 2. 
Herrick ’82 (Lyncodaphnia macrothroides). 
Sub-rectangular, greatly elongated, truncate behind, with a slight 
spine above; head and eye small, fornix moderate, beak truncate; an- 
tennules rather long, slightly curved, tapering a little toward the end, 
whence spring three lanceolate spines and several sensory filaments, 
five stout spines behind, above the middle, and several more slender 
ones; Swimming antennée very long, terminal sete smooth to the joint; 
labrum as in Daphnia; mandible attached behind a salient angle of the 
front margin of the shell; no abdominal processes; post-abdomen broad 
above, triangular; terminal claws pectinate, furnished with one very 
large toothed accessory spine and a smaller one; the first foot has a 
hook; the last foot consists of a large oval plate which bears poste- 
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