222 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
Shape of the nine spines above the anus is different. The head is con- 
vex, resembling J. sordidus most nearly, but the antennules are much 
longer and more slender than in any other known species. They are 
0.17 mm. long and about 0.016 mm. wide, while the longest seta is 
0.084 mm. long. The antenn are almost exactly as in J. sordidus. 
The labrum has the usual shape, as have the jaws and other append- 
ages. The margins of the shell are ornamented with spines simply 
pectinate or barbed as in J. agilis. In I. sordidus these spines are 
variously branched, and in that form alone of the European species, 
according to Kurz, is there a failure to perfect the moult; in our spe- 
cies, which has simply pectinate sete, the oid coverings are all but 
uniformly retained. The spines of the Jower posterior margin are 
from 0.16 mm. to 0.20 mm, long. 
Such are some of the chief peculiarities of the species, but, to make 
the relation between the four species of this little known genus even 
clearer, if possible, the following comparative table is appended. 
Ve agilis. 
esol) moulted periodiealliy.s..2ss.25 tv. cbcaecensssase¥essstcasedetene cease 
I, acutifrons. 
f { I. sordidus. 
Mie wnellenotimonulteduputeretammeds sw ccesseseeseencceeceetone teense eee Lit Soihipee 
I sordidus, 
Antennules not more than eight times as long as broad............seeeeee I. agilis. 
I. acutifrons. 
Antennules more than eight times as long as broad..........sscseeseceeeees I. spinifer. 
OW; P : { I. sordidus. 
@laywot post-abdomen: pectinatesc..css-cec: sseret cee ce veseesecoenceaseeeee eae \T., acutifrons. 
P I, agilis. 
Claw of post-abdomen not pectinate ..........c0+..sovesssosensccasssesssacosens L. spinifer. 
ASSLLONS Spine in! front OLiGlawpece.ssstsvacesessecheacetoeeeeedesee este eet eeaeeee I. acutifrons. 
I. sordidus. 
Bing: bristles or none in front off Cla w.....2csseses csearsatsscunp teow tccsaserapeceas I. agilis. 
I. spinifer. 
ZAATIIIS OPENID CMEAT THEI CLAW Sa. a. cccteacececcadeccee neces cnemee meee ee toeee ete aaae I. acutifrons. 
I. sordidus. 
Anus about midway of the posterior border. ....-............ecsceeseceeeseeees I. agilis. 
I. spinifer. 
Marginal'spines of shell much branched. «......22s40%.sscaesseewsvauewteacete st I. sordidus. 
I. agilis. 
Marginal spines nowhere much branched.............0:ssecssovsecceecanennens I. acutifrons. 
I. spinifer. 
Elongated anal spines on either side, tour or five, very long.............. I. spinifer. 
I. agilis. 
Elongated analspines more NUMETOUS «....;....sssccasocestesese+cseesvavtdocns {i acutifrons. 
I. sordidus. 
Upper (one to three) spines of the supra-anal series modified and { J. acutifrons. 
CWVAT LOW es Jos se cmsn> ss -osieicde cect sccvseeseeneseaeue dora sce tiacdeeam ine mentomaeacs I. agilis. 
Upper'spines like the others... :..scve. scronsecvesuiucs te ensea denchbe. theemameastes i peas 
