INTRODUCTION 
Freshwater Daphniidae of the genus Simocephalus Schédler, 1858 
are common in littoral aquatic vegetation all over the world. These 
‘tailless water fleas’ have been known since the middle of the 18th 
century (Schaeffer, 1755), but their taxonomy remains unsettled, 
with 61 specific and subspecific names proposed. Morphological 
variability is poorly known. This makes the taxonomic status of 
certain forms doubtful, since they may not represent taxa, but 
merely morphological varieties. The descriptions of numerous spe- 
cies are inadequate. Furthermore, some species which are supposed 
M.J. ORLOVA-BIENKOWSKAJA 
to be cosmopolitan, pantropical efc. are in fact groups of closely 
related species, with restricted distributions. Obviously, a world- 
wide revision of Simocephalus is necessary. Such an attempt is 
made here. 
The genus Simocephalus has been divided into four species 
groups: S. (vetulus), S. (exspinosus), S. (serrulatus) and S. (latirostris) 
(Orlova-Bienkowskaja, 1993a). The diagnostic characters of the 
groups are stable and well-expressed in all representatives. Interme- 
diate forms are absent. Furthermore, different characters are 
congruent, that is, they combine species into the same groups. Thus 
the species groups are given the rank of subgenera. 

Fig. 1 Morphology of Simocephalus. a—abdomen, ab — anal bay, ae —aesthetes, at — anal teeth, al — antennule, a2 — base of antenna (antenna is not 
shown), d—denticles of inner surface of ventro-posterior valve angle, dap — distal abdominal process, dd —denticles of ‘orsal valve margin, dh — 
depression of head shield between head and valves, dpva — dorso-posterior valve angle, dv — point of divergence of valves, dvm — dorsal valve margin, e — 
eye, f —fornices, fr —frons, g — gut, h—heart, hp — the place of head pores, 0 —ocellus, p—postabdomen, pap — proximal abdominal process, pe — 
postabdominal claw, pe — parthenogenetic eggs, ps — plumose setae of inner surface of ventral valve margin, pym — posterior valve margin, r—rostrum, s — 
setules of inner surface of posterior valve margin, sa — supra-anal angle, sp — sensory papilla of antennule, ss — sensory setae, vhm — ventral head margin, 
vvm — ventral valve margin, 1st — 1st trunk limb, 2nd — 2nd trunk limb, 3rd — 3rd trunk limb, 4th — 4th trunk limb, Sth — 5th trunk limb. 
