314 KKPORT OF COMMISSIONER OF FISH AND FISHERIES. 



are variously modified in the different genera and in tlie sexes, so that 

 much confusion has been introduced into the family by mistaking sexual 

 for generic modifications of these organs. The branchial pleopods are 

 usually protected by a thickened anterior pair, which, especially in the 

 females of our marine species, may be consolidated into a single opercu- 

 lar plate, as will be further described. The incubatory pouch in the fe- 

 males does not appear to extend farther back than the fourth thoracic 

 segment, and it may be confined to the second, third, and fourth seg- 

 ments. 



In the last-mentioned, as well as in many other characters, this fam- 

 ily is closely related to the next, and perhaps the Munnopsidw may yet 

 require to be united with it. Our species of the two families are at 

 once distinguished by the last three pairs of legs, which are ambulatory 

 in the AselUdce and natatory in the Munnopsidce. Our Munnopsidce are, 

 moreover, like the other known species of that family, destitute of eyes, 

 whOe the marine AselUdce have evident or conspicuous eyes, but the 

 fresh-water genus Gcecidotea Packard* is blind, as are also certain 

 foreign species referred to the present family. The relations of the 

 AselUdce with families other than the Munnopsidce are less evident. 

 They were associated by Professor Danat with his ArmadilUdce and Onis- 

 cidce to form his subtribe Oniscoidea, and, Idmnoria being excluded, the 

 group appears to be a natural one. 



Asellus communis Say, confined to fresh waters, and the only known 

 JSTew England representative of the genus, was described and figured by 

 the present author, in Professor S. I. Smith's "Crustacea of the Fresh 

 Waters of the United States," published in part II of this report (page 

 657, jilate I, figure 4). Our marine representatives of the family may 

 be most easily recognized by the consolidated pleon, ambulatory or pre- 

 hensile legs, none of them natatory, and the slender, elongate antennae. 

 The genera may be distinguished by means of the following table : 



C flattened above ; uropods 



short, subrudimcntary JiERA, p. 314 



Pleon < ' ■"■'-'11 developed Janiua, p. 319 



(vaulted; head large Munna, p. 335 



Jsera Leach. 

 Jcera Leach, Ed. Encyc, vol. vii, p. "434" (Am, ed., p. 273), "1813-14." 



Antennulse short, few-jointed; antennte moderately elongated ; man- 

 dibles with j)alpi ; first pair of legs similar to the following pairs ; lateral 

 margins of the thoracic segments projecting over the bases of the legsj 

 uropods short, rami subrudimentary; pleon protected below in the fe- 

 males by a subcircular plate. 



The short uropods and projecting lateral margins of the thoracic seg- 

 ments serve to distinguish this genus from its allies, and other charac- 

 ters of generic importance could doubtless be drawn from the pleon and 

 its appendages, as well as from other parts of the structure, but, as it 



'American Naturalist, vol. v, p. 751, figs. 132, 133, 1871. 

 tAm. Joiir. Sci.. II. vol. xiv, p. 301, 1862. 



I 



