76 TWELFTH AKNUAL REPORT. 



VIII. Genus Ilyockyptus. 



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; 

 setae terminal, but one seta near the base; the four-jointed ramus 

 of the antenna with but three (terminal) setee; six pairs offset, last 

 pair rudimentary; tail large, as in Lyncodaphnia, anus elevated; 

 intestine straight, without caeca, 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 aiiimal 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 cnly I. sordidus agree 

 that Ilyocryptus can not swim, but poles along in the mud on the 

 bottom by means of antenna and abdomen; our I. spinifer, on the 

 other hand, swims freely till loaded up with old clothes and filth. 



This genus is also closely allied with the Lynceidse. 



Sp. 1. Ilyocryptus sordidus, Lievin. 



(Plate C. Fi^s. 15, 16, 17.) 



Acantlwcercus sordidus, lievin, i.kydig. 



Ilyocryptus sordidus, sars, nokmann, p. e. mueller, kurz. 



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; antennae 

 short: four-jointed rami with no lateral seta3; 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 

 described such ceeca was an error; see Notes on Cladocera of Minn., 

 p. 246). 



