190 SMITHSONIAN MISCELLANEOUS COLLECTIONS [voL. 48 
long as the basal plates. The marginal teeth have a small ectocone 
and the mesocone becomes longer.”’ 
In a letter Dr. Pilsbry adds: “ This is the most interesting thing 
which has turned up in Mexico since Metostracon, . . . Your sur- 
mise that it was a Urocoptid turns out to be correct. The very short 
kidney, scarcely longer than the pericardium, alone settles it. These 
organs as well as the jaw and teeth are exactly as in Holospira, next 
to which it evidently belongs.” | 
XANTHONYX POTOSIANA n. sp. 
(PLaTE XLIV, FIGURES I, 2, 7) 
Shell thin, inflated, white, covered with a conspicuous straw- 
colored translucent periostracum ; whorls three, the first with minute 
radial riblets broken up into wavy segments or even radially dis- 
posed granules, with faint traces of microscopic spiral striation ; 
subsequent whorls faintly spirally striate and with the lines of 
growth irregularly moderately prominent; last whorl much the 
largest; suture deep, spire somewhat dome-shaped; aperture with 
the margin thin and sharp except on the pillar where there is a 
thin layer of milky white callus; plane of the aperture oblique, the 
basal margin passing imperceptibly into the arcuate pillar gyrate 
about a pervious axis; interior of aperture white. Max. diameter 
of shell 18, min. do. 12, height 14, the aperture 14.5 wide by 13.3 
mm. high. 
Habitat.—State of San Luis Potosi, on the Alvarez Mountains, 
at a height of 7,200 feet. 
This is the largest and most turbiniform species of Xanthonyx 
known. None of the specimens contained the animal, though ten 
of the shells, of various ages, were obtained. 
The type is No. 110,396 U. S. Nat. Museum. With this species 
were found Epiphragmophora pressula Morelet, Microceramus 
mexicanus von Martens, a Holospira which appears to be the true 
H. piloceret Pfeiffer, Glandina bellula Crosse and Fischer, and 
another form which is perhaps a variety of it, beside young spec- 
imens of a Glandina too immature to be positively identified. 
STREPTOSTYLA POTOSIANA n. sp. 
(PLatE XLIV, FIGURE 4) 
Shell moderately large and solid, opaque white with a brilliantly 
polished translucent yellowish-olive periostracum, with occasional 
darker zones axially arranged; the periostracum after the death of 
the animal rapidly peels off, leaving the surface white and smooth; 
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