346 BULLETIN 54, UNITED STATES NATIONAL MUSEUM. 



the basal joint of the legs, while in P. murdochi there is but one spine 

 on the basal joint. 



This species is also distinguished from J\ />//*///,/" in the presence of a 

 single median spine on the anterior part of the head, while in P. ////.xY/vV 

 there are two, one on either side of the median line and widely sepa- 

 rated; in the presence of two spines on the posterior part of the head, 

 while in P. hystriv there are four; in the absence of the double row of 

 spines on the terminal segment of the body; and in the absence of the 

 spine at the articulation of the third joint of the second pair of antennae. 



Family XIV. IDOTHEID^E. 6 



Body more or less broad, depressed. First pair of antennae with 

 the flagellum uniarticulate. Second pair of antennae with the flagel- 

 luni uniarticulate or multi-articulate. Mandibles without palps. 



Segments of thorax of uniform length and appearance. Epimera 

 sometimes distinct and sometimes coalesced with the segments. 



Abdomen with some or all of the segments fused to form the large 

 terminal segment. 



Legs usually nearly alike and ambulatory, but sometimes the three 

 anterior pairs are pronouncedly subcheliform in structure. 



Incubatory pouch normal. 



ANALYTICAL KEY TO THE GENERA OF THE FAMILY IDOTHEID.E. 



a. Sides of head emarginate or cleft and laterally produced beyond the eyes, which 

 are dorsally situated. First three pairs of legs with the sixth article or propodus 

 dilated and forming, with the reflexible dactylus, a subchelate hand. 

 b. Palp of maxillipeds composed of five articles. Inner branch of uropoda minute. 



Species large Genus Mesidofea, new genus 



I/. Palp of maxillipeds composed of three articles. Inner branch of uropoda half 



as long as outer branch. Species small Genus Chiridolea Harger 



a'. Sides of head in a dorsal view entire and not laterally produced. Eyes lateral. 

 Legs all nearly alike, with the sixth article or propodus not expanded or but 

 little expanded; seventh article prehensile. 



b. Flagellum of second pair of antenme well developed and multi-articulate. 

 c. Abdomen (including the terminal segment) consisting of three segments with 

 lateral sutures of another partly coalesced segment. Epimera of all the 

 segments, from the second to the seventh, inclusive, well developed and 

 distinct from the segments. 



d. Palp of maxillipeds with four articles Genus IdotJien Fabricius 



d' '. Palp of maxillipeds with five articles Genus Pentidotea, new genus 



c'. Abdomen consisting of one segment, with lateral sutures of another partly 



coalesced segment. 



d. Palp of maxillipeds with three articles. All the epimera coalesced and 

 perfectly united with the segments Genus Synidotea Harger 



a Arcturus hystrij- Sars. 



& See Sars for characters of family, Crust, of Norway, II, 1899, pp. 78-79. 



