188 On Ancient Water-fleas of the — [Mgnthly Microscopical 
Trise III. Limnadoidea [Bivalved]. 
Family. Limnadiade. 
Genus. Limnadia. Genus. Limnetis. 
Estheria. Leaia.* 
OrpeER II. Cormostomata. 
Suz-Orver 1. Peecilopoda (Caligus, &c.). 
2. Pycnogonoidea (Cyamus, &c.). 
Orpver III. Merostomata. 
Sus-Orver 1. Eurypteride * (Pterygotus,* Eurypterus,* &c.). 
2, Xiphosura (Belinurus,* Prestwichia,* Limulus, Neolimulus,* 
&c.). 
Sus-Ciass 4, Crrripepia. , 
5. Roraroria. 
Tue LEPERDITIADE. 
1. Leperditia (Fig. 17) is one of the largest and most com- 
mon forms of the Paleozoic Bivalved Entomostraca. Indeed it is 
found only in the Lower and Upper Silurian, Devonian, and Car- 
boniferous strata of different parts of the world. The earliest species 
seem to be L. Canadensis and L. Anna, small and variable, of early 
Silurian age. In the Upper Silurian strata much larger forms occur, 
especially L. Balthica, often almost an inch long, and sometimes 
larger. About twenty-four described species are recorded from the 
Silurian rocks. Others are mentioned, but the determinations appear 
uncertain. Three or four species are known from the Devonian rocks, 
chiefly of North America. In the Carboniferous period L. Okeni 
played an important part, both with a goodly growth in the open 
seas, and as dwarfed forms, with one variety or more for every 
locality, in the salt-marshes and lagoons among the coal-jungles, 
where layer after layer of the old mud-beds abound with the cara- 
paces of these little water-scavengers of the period. Three or four 
other species of Leperditia are known in the coal-shales of Scotland. 
This genus was instituted in 1851 by M. Rouault. Its bivalved 
carapace is smooth, convex, horny in appearance, often brownish, 
sub-oblong or somewhat semi-ovate in outline, reminding one of 
some beans and tamarind-seeds; longer than broad (or high), in- 
equilateral; posterior half broadest ; dorsal border straight; ventral 
border nearly semicircular ; anterior and posterior borders oblique 
above, rounded below, the valve-margin passing from each end of 
the hinge-line in an oblique direction downwards and outwards to 
about half the breadth of the valve, where it meets the curved ends 
of the ventral border, and so forms the more or less angular ex- 
tremities of the valve, the former of which is narrower and sharper 
than the latter. The valves are united along their upper (dorsal) 
