250 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
the spire. The middle one of these keels is decidedly stronger than the 
rest and marks the widest part of the turns, while the anterior one is the 
least developed on all the early turns. On the last whorl the three are 
subequal. In addition to the spiral keels the spire is marked by slender, 
axial riblets, of which there are about eighteen upon the first of the 
remaining and twenty upon the penultimate turn, the intersection of the 
riblets and the spiral keels forming the tubercles. The tubercles slope 
convexly toward their anterior limit and are somewhat excavated pos¬ 
teriorly. Periphery of the last whorl marked by a sulcus. Base with two 
equally strong keels on the posterior half, separated from each other by a 
sulcus as wide as the peripheral one, and a third weak thread on the base 
of the thick columella. Aperture irregular, the main portion subcircular, 
strongly channelled posteriorly and anteriorly; outer lip moderately strong, 
columella short, stout, and decidedly twisted, covered partly by the very 
strong purplish parietal callus. Length, 5.2; diameter, 2.2 mm. (Bartsch.) 
Type in United States National Museum, No. 195377. Type locality, 
San Pedro, California. 
Range. San Pedro to San Diego, California. 
Trifora pedroana Bartsch, 1907 
Plate 71, fig. 1 
Proceedings of the United States National Museum, 33:250; PI. 16, fig. 1. 
Shell sinistral, elongate-conic, light brown. Nuclear whorls four, 
increasing regularly in size. The first three smooth, probably by erosion, 
the next faintly, obsoletely sculptured by about thirty-six slender, equally 
strong and equally spaced axial riblets and two slender spiral threads, 
separated by a narrow channel near the periphery of the whorl. Succeed¬ 
ing turns separated by broad and deep channeled sutures; the first seven 
ornamented by two equally strong spiral rows of tubercles, which are 
separated by a channel, as wide as that at the sutures. Beginning with the 
eighth turn, a slender tuberculate keel makes its appearance in this channel, 
situated a little nearer the posterior keel than the suprasutural one. This 
keel increases steadily in size and on the penultimate turn exceeds the 
supraperipheral one in strength. On the last three turns the tubercles of 
the posterior keel are a little more strongly developed than on the other 
keels. On all the keels the tubercles slope a little more abruptly at their 
posterior border, the anterior edge being gently rounded. There are fifteen 
tubercles upon the first, seventeen upon the fifth, and twenty-four upon 
the penultimate post-nuclear turn. The tubercles are connected spirally 
by a moderately wide band and axially by slender riblets, the spaces in¬ 
closed between these connections appearing as rounded pits. The entire 
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