M Y S I D E A. 653 



Carapax more or less rostrate. Inner antennae having two flagella 

 and an oblong scale at apex of base. Eyes symmetrical, rather 

 short. Thoracic feet sixteen, all having a natatory palpus, which 

 is multiarticulate, tarsus of pediform branch not jointed, unguicu- 

 late. Abdominal feet mostly small and imperfect, fourth pair very 

 much elongate. [Last abdominal segment bilobate or emarginate 

 behind.] 



The elongate form of the fourth pair of abdominal legs may be 

 only sexual and possibly occurs in other genera. The sixteen tho- 

 racic feet, and the imperfect character of the larger part of the abdo- 

 minal feet distinguish this genus from Pi-omysis. The inner caudal 

 lamellae have the transparent globule at base, characteristic of Mysis. 

 The females have the usual pouch under the posterior part of the 

 thorax. 



Themisto, GOODSIR, Edinburgh New Phil. Jour., xxxiii. 176, pi. 2, f. 4; Goodsir's 

 T. brevispinosa corresponds well with our species in general form ; but his figure of the 

 T. longispinosa is quite another thing in its legs, which to us are unintelligible. 



Macromysis, A. WHITE, Cat. Crust. Brit. Mus., 1847, 81, and 1850, 45. 



Mysidium, DANA, Am. J. Sci. [2], ix. 130, 1850. 



MACROMYSIS GRACILIS. 



Carapax brevissime* restrains, segmentum thoracis septimum non tegens, 

 posticb quoad doi'sum profunde excavatus et latere rotundatus. Basis 

 antennarum internarum tenuis ; flagella valde inmqua, superiore fere 

 duplo longiore et dimidium corporis longitudine valde superante. An- 

 tennae externce fere corporis longitudine, squamd basali lanceolatd. 

 Segmentum abdominis posticum apice obsolete rotundato-bilobatum et 

 spinulis ciliatum, lamella caudali externd plus dimidio brevius; hoc 

 lamella extus spinulosd, valde longiore quam interna. 



Carapax very short rostrate, not covering last thoracic segment, be- 

 hind over the back profoundly excavate and laterally rounded. 

 Base of inner antennae slender, flagella very unequal, the superior 

 nearly twice the longest, and much longer than half the body. 

 Outer antennas almost as long as the body, basal scale lanceolate. 

 Last abdominal segment obsoletely rounded bilobate at apex and 



164 



