CLASS GASTROPODA 
247 
tubercles, the posterior row being a little more strongly developed than the 
anterior. These rows are separated by a channel about as wide as the 
tubercular ridges. Beginning with the fifth whorl, a slender tuberculate 
cord appears in the middle of the channel which separates the two spiral 
ridges; this grows steadily in size with each turn, until on the last volution 
it is quite as strong as the suprasutural cord. On the later turns the middle 
and suprasutural spiral cords and their tubercles fall off quite abruptly at 
their posterior border and slope roundly toward their anterior limit. The 
tubercles on all the turns are joined by rather wide spiral bars and axial 
ribs, which inclose deep squarish pits between them. Periphery and base 
of the last turn well-rounded, the former marked by a low, somewhat 
flattened keel, which is separated from the supraperipheral cord by a 
channel as wide as the one which separates the supraperipheral keel from 
the middle one. The channel is crossed by the extension of the axial 
riblets. The base is marked by two spiral cords about as broad and of the 
same character as the peripheral one. These cords are separated by 
channels as broad as that which separates the peripheral cord from the 
supraperipheral one and are crossed by feeble extensions of the axial 
riblets. The anterior basal cord is situated upon the base of columella and 
its anterior border fuses almost imperceptibly with it. The entire surface 
is crossed by many exceedingly fine spiral striations and lines of growth. 
Aperture subquadrate, posterior angle obtuse, anteriorly strongly chan¬ 
neled ; outer lip subpatulous anteriorly, not sinuous; columella short, thick 
twisted. Columella and the parietal wall are covered with a strongly devel¬ 
oped callus. Length, 4; diameter, 1.5 mm.; but the type has lost three of 
its nuclear whorls. (Bartsch.) 
Type in United States National Museum, No. 106424. Type locality, 
Point Abreojos, Lower California. 
Range. San Diego, California, to Point Abreojos, Lower California. 
Trifora catalinensis Bartsch, 1907 
Plate 71, fig. 18 
Proceedings of the United States National Museum , 33:253; PI. 16, fig. 18. 
Shell sinistral, elongate-conic, rather stout, with the posterior half of 
the exposed portion of the whorls white and the anterior half light brown. 
(Part of the nuclear whorls decollated.) The three and one-half remaining 
turns of the nucleus increase regularly in size and are marked by about 
twenty-six slender axial riblets on the first, and thirty-two on the next 
and on the third whorls. In addition to these riblets the nuclear whorls 
are encircled by two prominent sublamellar slender, finely tuberculate 
spiral keels, which are placed about equally distant from the sutures and 
[ 545 ] 
