176 PALEOZOIC PALEONTOLOGY. 
The dimensions of a large specimen are: maximum diameter, 21 
mm.; width of aperture, 18 mm. 
Remarks.—In none of the New Jersey specimens have the delicate 
surface markings of this species been observed. These markings 
should consist of fine transverse and still-finer revolving lines, giving 
to the surface of the shell a cancellated appearance. 
TETRANOTA BIDORSATA (Hall). 
Plate XII., Figs. 18-19. 
1847. Bucania bidorsata Hall, Pal. N. Y., vol. I., p. 186, pl. 40, 
figs. 8 a-q. 
1897. Tetranota bidorsata U. & S., Pal. Minn., pt. IL, p. 877, pL 
65, figs. 10-18: 
Description — ‘Shell usually about 12 mm. in height, but the height 
may exceed 20 mm., and occasionally reaches 25 mm.; volutions two 
and a half to three and a half, vertically compressed, sublunate in 
section; the width for the inner volutions or in young specimens a 
little greater than twice the height; in old examples the increasing 
altitude of the centro-dorsal ridges causes the width just behind the 
aperture to be proportionally somewhat less; umbilicus large, deep, 
rather sharply defined, the width generally about half the greatest 
diameter of the shell; the latter dimension is to the greatest width 
of the aperture about as three is to four. Aperture somewhat abruptly 
expanded laterally, the height and width about as three is to seven; 
slightly indented by the preceding whorl; lips thin, the outer one, 
with a moderately-deep emargination, taking up between one-fourth 
and one-third of the anterior outline; depth of same about one-fifth 
less than its width. Dorsum with four strong, revolving ridges, the 
two central ones nearer each other than to the lateral ones and higher, 
the altitude, also, increasing gradually to the aperture; between them 
lies the broad slit-band, which is more distinctly concave on the shell 
than on internal casts, the double ridge in the latter, particularly 
near the aperture, often appearing as a broad and more or less flat- 
topped, single ridge; on each side of the central ridges there is, first, 
a broad groove, then an obtusely angular ridge, and finally a narrower 
