cot(x)=1/tan(x)
x(u,v) = D*(A*sin(b)*cos(u)+R*cos(v+phi)\
         *cos(u+W)-R*sin(m)*sin(v+phi)*sin(u+W))*exp(u*cot(a))
y(u,v) = (-A*sin(b)*sin(u)-R*cos(v+phi)\
         *sin(u+W)-R*sin(m)*sin(v+phi)*cos(u+W))*exp(u*cot(a))
z(u,v)=(-A*cos(b)+R*sin(v+phi)*cos(m))*exp(u*cot(a))
D=1;a=pi/180*88.9;b=pi/45;phi=11*pi/36;m=pi/180;
W=-pi/90;A=22.2;R=1.2
#
set style line 100 lw 0.05  lc rgb "dark-green"
set pm3d depth hidden3d ls 100
set palette defined (0 0.8 0.5 0, 1 1 1 0.5)
unset key
unset border
unset tics
unset colorbox
set parametric
set ticslevel 0
set size square
set view 65,30,3.5
set vrange[-7*pi/4:pi/4]
set samples 256
set isosamples 24
L=20
set xrange[-L:L]
set yrange[-L:L]
set zrange[-1.7*L:0.3*L]
set urange[-26*pi:0]
set cbrange[-2*L:0]
splot x(u,v),y(u,v),z(u,v) w pm3d 
pause 3
set term pngcairo size 480,480
set out "vs_turritella.png"
replot