42 The American Geologist. January, 1894 
joints. In general form, it is slightly crescentic, with the 
margins thickened, the anterior one being strongly crenu- 
lated. Long seta- extend posteriorly from the crenulations 
on the dorsal side of the leg, making a conspicuous fringe 
along the distal half of the exopodite. 
Plate in, figure T, represents a dorsal view of Triarthru* 
Becki, showing the antenna? and the exposed portions of the 
appendages. The antennae and legs on the right side arc 
drawn from one specimen, and the legs on the left side are as 
shown in another individual. The biramous character of the 
entire series of thoracic legs is very evident, as is also the 
distinction between the crawling and swimming members. 
Figure 8 shows the right second and third legs of the free 
thoracic segments. In figure 9, the upper exopodite is rep- 
resented without setae, so as to bring out the structure in 
greater detail. On the lower leg, the seta? are shown. 
The antenna? are about as long as the head, and are com- 
posed of short conical joints. They usually occur in the po- 
sition shown in figures 5 and 7. but occasionally lie close to 
the margin, figures 3 and 4, and sometimes curve backwards 
over the head, as in figure 6. 
It is not necessary in this place to describe in detail the de- 
velopment of Triarthrus Becki, but attention ma}" be called to 
two early larval forms. The youngest is shown on Plate in, 
figure 1, and may be compared with the first segmented stage, 
figure 2, and with the adult, figure 7. At this early stage, the 
animal is less than one millimeter in length (.63 mm.), and 
has no distinct separation into parts. The division into a 
cephalic and a caudal region is indicated by a transverse groove r 
but as 3 T et the body segments are undeveloped. After the sep- 
aration of the head and pygidium, the thoracic segments are 
introduced successively between the head and abdomen until 
the full number is reached, and the animal measures from 10 
to 55 millimeters in length. The segmented stages have been 
described fully by Walcott,* and an outline figure of the stage 
with one thoracic segment is given in figure 2. 
The final conclusions to be reached from a complete study 
of the development and structure of these animals can only as 
yet be surmised. It is quite evident, however, that they are 
♦Transactions Albany Institute, vol. x. 
