86 University of Texas Bulletin 



is broad and very steep. On the cast each whorl shows several 

 moderately deep constrictions which on the flanks have a radial posi- 

 tion, but on the ventral portion curve strongly forward. The cross 

 section of the adult specimens is subtrapezoidal, the flanks being flat 

 (pi. II, fig. 41-42) and the ventral portion not very strongly rounded. 

 The height is much greater than the width; the greatest width exists 

 near the umbilical border. In the adolescent stage the flanks are not 

 quite so much flattened (pi. II, fig. 38-40) and the ventral portion more 

 rounded, the whorl being broader than high. In the young specimens 

 (pi. II, fig. 32, 36) the cross-section is semilunar, flanks and ventral 

 region forming an uninterrupted curve, the greatest .width still at the 

 umbilical shoulder ; but the whorl is much broader than high. 



Several specimens preserve the ornamentation. As does everything 

 else in this species, this ornamentation changes with the stages of age. 

 A very small specimen (pi. II, fig. 28, 29) with about 7.5 mm. dia- 

 meter shows about twenty-two transversal ribs on the umbilical shoul- 

 der. These are broad at their base, sharper but rounded at the top; 

 they are strongest below the umbilical shoulder and disappear on the 

 flanks, and are separated by deep interstices about as broad as the 

 ribs. The flanks and ventral portion show about sixteen to seventeen 

 fine spiral ribs between the umbilical shoulder and the sipho: similar 

 ribs also pass at least over the upper part of the transversal ribs and 

 are best visible in the interstices. The spiral ribs are rounded and 

 separated by narrow furrows. 



On a specimen of about 26.5 mm. diameter (pi. II, fig. 38-40), the 

 transversal ribs on the umbilical shoulder begin to disappear ; they are 

 slightly inclined forward, are very short, show only on the umbilical 

 shoulder and on the upper part of the umbilical wall, and disappear at 

 the beginning of the flank. They are low, rounded and separated by 

 rounded furrows which are Narrower than the ribs. These latter ap- 

 pear to be about thirty-five on the whorl. On the umbilical shoulder, 

 the flank and the ventral region, we count between the umbilical bor- 

 der and the sipho, thirty-one spiral ribs. The first two of these at the 

 umbilical shoulder are separated by a wider interstice; all the others 

 are practically at equal distances from each other. The ribs are 

 rounded and separated by very narrow furrows. These spiral ribs 

 cross the transverse ones in their whole length. The spiral ribs are 



