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



riorly the ordinary branchial coil, here shaped like a thumb and fore- 

 finger. The young is of a different shape and bears a long spine. 

 The male is known from figures recently published by Birge ('91). 



GENUS ILYOCRYPTUS. 



. Form compact, short; head short, triangular, with large fornices 

 forming a roof over the head; the posterior margin of shell nearly as 

 long as the inferior; lower angle a broad curve; antennules two-jointed, 

 basal joint very short, second joint straight, rather long; setfe termi- 

 nal, but one seta near the base; the four-jointed ramus of the antenna 

 with but three (terminal) setse; six pairs of feet, last pair rudimentary; 

 tail large, as in Lyncodaplinia, anus elevated; intestine straight, with- 

 out cjeca, but an expansion near the rectum sometimes simulates one; 

 the margin of the shell is bordered with long spines, which may be 

 branched or simply pectinate. There is often, perhaps generally, a 

 failure to entirely remove the moulted shell; when this occurs the 

 newly- formed shell from each moult remains under the older ones till 

 the animal seems to be wearing six or more overcoats, and the spaces 

 so formed become filled with algse and filth till the animal is no longer 

 able to swim. P. E. Mueller and Kurz, who seem to have seen only 

 I. sordidus, agree that Ilyocryptus cannot swim, but poles along in the 

 mud on the bottom by means of antennae and abdomen; our I. spinifeVy 

 on the other hand, swims freely till loaded np with old clothes and 

 filth. 



This genus is also closely allied with the Lynceidce. 



* Ilyocryptus sortlidus Lievin. 

 Plates LV, Figs. 6, 10, 11; LVI, Figs. 15-17. 

 Acanthocercus sordidus — Lievin, Leydig. 



Body higher than long; head small, terminating anteriorly in 

 almost a right angle; posterior part of the shell margins covered with 

 branching, thorny spines; antennules cylindrical; antennse short; four- 

 jointed rami with no lateral setfe; post abdomen large, broad; terminal 

 claws with two spines at the base; anus in the middle of the posterior 

 margin, which is very heavily armed with spines; a hairy abdominal 

 process is present, according to Kurz. 



There are no anterior cteca (my statement that P. E. Mueller de- 

 scribed such cfEca was an error; see Notes on Cladoreca of Minnesota, 

 p. 246). Length 0.78 mm. The tabulated comparison beyond makes 

 a full description unnecessary. Birge reports this species from Wis- 

 consin and it has been encountered in Minnesota. 



