ISOPODS OF NORTH AMERICA. 3 



natatory, but their function is respiratory for the most part. Some 

 or all of the segments of the abdomen may be united. The telson is 

 very rarely free. 



Marsupial plates are developed in the female, forming an incubatory 

 pouch. 



ANALYTICAL KEY TO THE TRIBES OR SUPERFAMILIES OF NORTH AMERICAN I8OPODA. 



a. Legs of first pair cheliform. Uropoda terminal. Pleopoda, when distinctly 



developed, exclusively natatory I. TANAIOIDEA or CHELIFERA 



a' '. Legs of first pair not cheliform. 

 b. Uropoda lateral. 



c. Uropoda forming together with the terminal segment of the abdomen a 

 caudal fan. Pleopoda for the most part natatory. 



II. CYMOTHOIDEA or FLABELLIFERA 

 cf . Uropoda valve-like, inflexed, arching over the pleopoda, which to a great 



extent are branchial III. IDOTHEOIDEA or VALVIFERA 



b'. Uropoda terminal. 



c. Pleopoda not fitted for air breathing, exclusively branchial. 

 d. Pleopoda generally covered by a thin opercular plate (the modified first 



pair). Free forms , IV. ASELLOIDEA or ASELLOTA 



d' . Pleopoda never cqvered by an opercular plate. Parasitic forms. 



V. BOPYROIDEA or EPICARIDEA 

 c'. Pleopoda fitted for air breathing VI. ONISCOIDE A 



I. TANAIOIDEA or CHELIFERA. 6 



Head fused with the first and sometimes with the second segment of 

 the thorax to form a carapace. Branchial cavity on each side of cara- 

 pace. Following five or six segments of thorax distinctly defined with 

 epimera small or inconspicuous. 



Uropoda terminal, consisting of a short basal segment and one or 

 two filamentary branches. First pair of legs terminate in a cheliform 

 hand. Abdomen generally composed of six segments, usually dis- 

 tinct. First pair of antennae furnished with one or two flagella. First 

 maxillae have a backward directed palp, situated in the posterior part 

 of the carapace. Maxillipeds have a four-jointed palp, and a large 

 backward directed branchial epignath, which passes into the branchial 

 cavity. Pleopoda, when present, natatory. 



ANALYTICAL KEY TO THE FAMILIES OF TANAIOIDEA OR CHELIFERA. 



a. Body scarcely attennuated behind. Mandibles without palp. Superior or first 

 pair of antennae with one flagellum (never two), which is sometimes absent, 

 usually rudimentary, rarely well developed in female, and multiarticulate in 

 male. Anterior maxillae with only a single masticatory lobe and a one-jointed 

 palp; posterior ones quite rudimentary. Second pair of legs ambulatory in 

 character. Epignath of maxillipeds narrow, falciform Family I. TANAIDJE 



a Sar's analytic key has been used with slight modifications. See his An Account 

 of the Crustacea of Norway, II. Isopoda, 1899, p. 3. 



& See Sars' Crust, of Norway, II, 1899, pp. 4, 5, and Norman and Stebbing, Trans. 

 Zool. Soc. London, XII, 1886, pp. 78-79, for characters of superfamily. 



