222 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



shape of the nine spines above the anus is different. The head is con- 

 vex, resembling I. sordldiis most nearly, but the antenuules 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 antennre are almost exactly as in I. 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 I. agiUs. 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 simplj' pectinate sette, the old coverings are all but 

 uniformly retained. The spines of the lower 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. 



The shell moulted periodically H' "^'^'^; 



The shell not moulted but retained H' «o':<^'.^'"«- 



{ J. spmifei: 



( I sordidus. 

 Antennules not more than eight times as long as broad < I. ac/ilis. 



\ I acutifrons. 

 Antennules more than eight times as long as broad /. spinifer. 



Claw of post-abdomen pectinate I {• ^'""'J'.f'"® 



Claw of post-abdomen not pectinate < / "^- '-^ 



^ ^ {1. spinifer. 



A strong spine in front of claw /. acutifrons. 



{I. sordidus. 

 I. agihs. 

 I. ypinifer. 



Anus opening near the claws /. acutifrons. 



!I. sordidus. 

 I. agilis. 

 J. spinifer. 



Marginal spines of shell much branched /. sordidus. 



!I. agilis. 

 I. acutifrons. 

 I. spiniftr. 



Elongated anal spines on either side, four or five, very long /. spinifer. 



(I. agilis. 

 I. acutifrons. 

 I. sordidus. 



ons. 



Upper (one to three) spines of the supra-anal series modified and f I. acutifrons. 

 enlarged \ /. agilis. 



Upper spines like the others < r' '^ •' S ' 



^^ *^ 1.7. spinifer. 



