640 ARTHROPODA SUB-KINGDOM VII 
is seldom well defined; but, on the other hand, the head is sharply demarcated 
from the rest of the body, and is usually provided with two pairs of antennae 
and two large eyes, in addition to which there is often a small unpaired eye. 
About the mouth are the large upper lip (hypostoma), two broad cornute 
mandibles without palps, one or two pairs of maxillae, and often a lower lip 
in the form of two elevations below the mandible. 
To the thorax are attached foliaceous, overlapping, biramous legs ; these 
usually occur in considerable numbers, and while rarely more than eight pairs, 
they may vary from four to forty, and become smaller posteriorly. They 
serve for swimming and grasping, and as a rule are supplied at the base with 
respiratory tubes. The abdomen is partly without legs and frequently ends 
in a segment bent down or recurved, and furnished with two claw-shaped or 
expanded caudal appendages. All Phyllopods have the sexes distinct. The 
males are usually much less numerous than the females, and the latter produce 
chiefly by parthenogenesis. 
Order 1. BRANCHIOPODA. Latreille. 
Ten to forty or more pairs of leaf-like feet ; carapace, when present, shield-shaped or 
bivalved ; in the latter case enclosing the whole body and capable of being closed by a 
transverse adductor muscle. 
Recent Branchiopods are almost exclusively inhabitants of fresh or brackish water, 
the exceptions being found in strong brine, as in Great Salt Lake, ete. The males are 
usually less numerous than the females, and in the case of some species several years 
may pass without their appearance, the females reproducing parthenogenetically. 
Family 1. Limnadiidae. Baird. 
Carapace bivalved, containing the whole body, and closed by a transverse adductor 
muscle. Antennae well developed ; eyes sessile ; thoracic legs in ten to twenty-eight pairs, 
the first one or two pairs in the male forming clasping organs. Abdomen small, 
without appendages, and terminated by a pair of caudal processes, 
Estheria, Riippel (Figs. 1332, 1333). Shell composed of two thin rounded valves, 
A B 

Fic. 1838. 
re 2a9 
d , BiG, 1962, Estheria, sp. indet. Lower 
HLstheria minuta, Alberti. Lettenkohle Dolo- barren Coal Measures; Carrollton, 
mite; Sinsheim, Baden. A, 1/;. B, 8/). C, Ohio. Umbonal portion showing 
Portion of the exterior, 5”/;. muscular or nuclear node, 1%/;, 
united by a straight toothless margin. External surface concentrically ridged or 
striated, and between the ridges are more or less regularly interlacing or branching 
