Drainage Systems of New Mexico. — Tarr. 261 
Figs. 2, 3, 4. — Agaricocrinus americanus Rcemer. 
Basal view of specimens having base but slightly depressed. Plates 
large and more or less prominent. Second radial in right postero-la- 
teral ray truncated byinterradial. 
Fig. 5. — Agaricocrinus amerif^anus Rcemer. 
Specimens with sixteen arms and showing the very deeply excavate 
form of the base. 
Fig. 6. Left postero-lateral ray of A. wortheni showing arrangement 
of plates : 
a. First primary radial — hexagonal. 
b. Second primary radial — hexagonal. 
c. Third primary radial — pentagonal. 
d. Secondary radials. 
e. Interradial plates. 
f. Arm plates. 
The first secondary radial on the posterior side of the postero-lateral 
ray is essentially a brachial piece with two rows of additional plates 
upon its upper slanting sides. 
Note. — The varying proportions of the first interradial in the above 
figures is especially noticeable. The anal area is directed upwards. 
Fig. 7. — Orthis keokuk Hall. Vental valve. 
The figures of this shell given by Hall were evidently from specimens 
more or less exfoliated. Well preserved specimens are rare. The 
above figure is from a specimen showing very prominent radiating 
striae and concentric lines of growth. The strire are fine and close up 
on the upper part of the shell but become abruptly coarser at each 
concentric line of growth. The plications are very prominent on the 
outer row. 
Length 3 in. Transverse diameter 4>^ in. Elevation at center IJ^ 
in. Width of hinge line ? 
DRAINAGE SYSTEMS OF NEW MEXICO. i 
By Ralph S. Tarr, Austin, Texas. 
In the arid climate of New Mexico the annual rainfall in the 
highest mountains is often more than thirty inches ; hut over 
the larger part of the territory on all the plains and plateaus 
it is generally not more than twelve inches. During the sum- 
mer months the country is liable to excessive rainfalls lasting 
for an hour or two and in that time doing great erosive work. 
Generally the erosion of a year is done in a few days. For the 
remainder of the year rains are rare and slight in'^xmount and 
the water that falls sinks readily into the parched soil without 
doing any appreciable work of erosion. For nine months of 
the year the wind erodes more rapidly than the rain water. 
New Mexico, being located within the area of the Rocky 
mountain uplift is topographically typical of that region. In 
^ In the preparation of this paper I am indebted to Prof. AVm. M. 
Davis for valuable criticism and suggestions. 
