cbagin.] DESCRIPTIONS OF SPECIES. 93 
whorls and only one of them approaches in size the smallest one of the 
types figured from the Sierra -de Catorce by Castillo and Aguilera. 
The cavity is oval in cross section, and its lesser diameter is equal to 
about four times the thickness of its wall. 
The following is a translation of the specific diagnosis : 
Shell small, free in the adult state, tubular, inrolled planorbiform, sinistral, 
last whorl unrolled. Tube rugose, bearing two spiral carinae at the base, the 
exterior more prominent than the interior, inclosing between them a narrow 
furrow. Aperture circular or slightly oval. Surface furnished with sinuous 
growth lines and quite pronounced on the upper part of the whorls, less so on 
the inferior part. 
INTATICID.E. 
Genus NATICA (Adanson) Lamarck. 
Natica williamsi sp. n. 
PI. XX, figs. 7, S. 
Shell of medium size in its genus, imperforate; whorls 5; spire 
small, short, and concave, giving it an apiculate aspect when well pre- 
served, its height about one-fifth of that of the entire shell ; body 
whorl large, obliquely inverted pyriform, flatfish on the region below 
the weakly impressed suture, the greatest convexity being somewhat 
above the middle of the whorl ; inner lip and columellar region cov- 
ered with a layer of callus; aperture rather long and narrow, narrow- 
ing gradually upward, rounded below, subacute above. 
Measurements. — Height, 40 mm. ; breadth, about 30 mm. Other 
specimens, not allowing precise measurement, indicate about one and 
a half times these dimensions. 
Occurrence. — A mile and a half east of Malone station, associated 
with Natica infecta. Nerita nodilirata, etc. At this locality it is one 
of the commonest of the Malone Gastropoda. Nearly a hundred 
specimens are in hand, most of them poorly preserved owing to 
weathering. 
This shell is named for Mr. John W. Williams, who in 1897 was 
prospecting in the Quitman Mountains not far from Malone, and 
whose exceed ingty kind and thoughtful interest in my work, dis- 
played in many practical ways, contributed not a little in that year to 
the pleasure as well as the success of my explorations in the Malone 
district. 
In its low and concave spire and feebly impressed suture, and a tend- 
ency to greater breadth in the upper part of the body whorl, the 
shell recalls Actseonella and constitutes an unusual phase of Jurassic 
Amauropsis. 
