704 A. T. EVANS 
water was very muddy and a broad incrustation of alkali ex- 
tended back several feet from the shore of the pool. In this pool 
the animals frequented the deeper parts, even the bottom. 
Occasionally they were seen gliding through the water at a depth 
of 2 or 3 inches where they were easily observed. Thamno- 
cephalus swims like Branchipus, with the dorsal surface of the 
body down, the head acting as the prow of a boat, the long 
thoracic appendages stroking regularly and rapidly. The ver- 
tical position in the water was easily changed by the elevation 
or depression of the broad, flat tail. Associated with Tham- 
nocephalus in this habitat were found Ambylstoma tigrinum, 
Bufo cognatus, tadpoles, snails, dipterous larvae of several kinds 
and beetle larvae; as well as large numbers of Phyllopods of the 
genera Apus, Streptocephalus and Estheria. 
Thamnocephalus platyurus is very similar in size and shape 
to Branchipus, except for the peculiar frontal appendage of the 
male and the broad, depressed, fin-like tail present in both sexes. 
The end of the tail is not forked, as in many of the other Phyl- 
lopods like Streptocephalus texanus, but is evenly rounded 
except for a slight but distinct notch in the extreme posterior 
part. The thickened posterior end of the abdomen occupies the 
central third of the tail, while the outer, lateral, portions, which 
comprise the rest of the tail, are thin and membranous. Each 
second antenna of the male is composed of a short, thick, fleshy 
portion, which forms the basal joint; and an outer, recurved, 
distal portion, composed of a more or less horny tissue. This 
outer distal joint is quite rigid, and when not in use, it is carried 
like a recurved tusk, backward under the body. All traces of 
an inner branch of the second antenna are lacking. In the female 
the second pair of antennae are long and oar-shaped, and near 
the distal end broaden laterally, rather abruptly, Just before the 
terminal point is reached. When not in use, the female carries 
these second antennae folded back under the head and thorax. 
In color these animals vary from a transparent to a milky 
white, specimens preserved in alcohol becoming more or less 
flesh-colored. The intestine is visible through the middle of 
the rather thick body, as a dark brown tube colored by the con- 
