TO THE EXISTING ARTICULATA. 41 



similar in circumference to the latter, and placed next to it (Fig. 12, L) ; and the other 

 smaller, and situated rather higher upwards (I). The gill is easily known by its bladder-like 

 form, by the want of muscles extending towards or penetrating into it, and by the absence 

 of fin-bristles at its margin ; while all the other lobes are surrounded both by fin-bristles and 

 are also supported by muscles, which latter extend towards them, and serve to assist in 

 locomotion. The lobes of the outer side, although the largest, are yet the most delicate 

 and the least protected ; they are, however, furnished with muscles, and they therefore 

 appear to be more intended for the protection of the gill, than for locomotion. This is 

 further confirmed by their inverted position in relation to the gill in the case of those genera 

 covered with shells, as well as in the naked genera. I would therefore call them "protecting 

 lobes." 



There is no doubt that the absence or presence of a shell is the main distinctive 

 character of the group ; and since a classification in which the natural characters are placed 

 in opposition becomes necessary, I have planned the following formula : 



First Group. 



Genera with SJicUs. 

 Cliaracfers. — Eyes immoveable, placed closed to one another. The rings of the body 

 partialhr spinous ; those of the abdomen likewise bear feet ; the last is a horny capsule, 

 furnished with various appendages. Gill of the feet attached above the protecting lobe. 



Division A. — Shell with two Flaps. 



Eyes united into a circular group ; anterior antennae having two rows of filaments, 

 posterior simple ; — all the antennae at least as long as the head. No accessory parts of the 

 mouth, fifteen abdominal rings, the moveable terminal spines unarticulated. 



Genera. — Limnadia, Estheria. 



Division B. — Shell, Shield-like. 



Eyes crescent-shaped, more distinctly separated ; the antennae scarcely recognizable ; 

 two pair of accessory parts of the mouth behind the jaws ; first pair of feet (not including 

 the real first, but rudimentary pair) furnished with four long, many-jointed fibres, instead 

 of the fin-lobes ; abdomen formed of more than fifteen rings, the terminal spine of the 

 last ring long, moveable, and articulated. 



Genera. — Lepidurus (with a flap between the end bristles), Apus (without this flap). 



Second Group. 



Genera without Shells. 

 Character. — Eyes pedunculated, moveable. Antennae unequal ; the anterior consisting 

 of simple fibres ; the posterior tongue-shaped, and serving as organs of copulation. No 

 accessory parts of the mouth behind the jaws ; the gills of the feet beneath the protecting 

 lobes ; abdomen without feet and smooth ; ovarian and seminiferous sacs, external. 



Genera. — Branchipus (abdomen furnished with nine joints, with two caudal fins), 

 Artemia (abdomen furnished with six joints, with two terminal lobes). 



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