Shell, Shell and Sand, Imperial
Imperial's Shell and Shell and Sand patterns are exactly the same except that the Shell and Sand has stippling between the shells as shown on the right. I suspect that when Imperial first released the Shell version it didn't sell well so they added the stippling to make the design more interesting. It's worth noting that some auctioneers may list either pattern as Shell, so check carefully.
Shell plates, 9 inches

Marigold, 275, 375 (both 2006),
   500 (2008), 160, 675 (both 2012),
   375 (2016)

Smoke, 275, 450, 525 (all 2013),
   225, 255, 600 (all 2014), 225, 275, 405 (all 2016)

Smoke, marigold over, 1,400 (2013)

Shell bowls, about 7-8 inches

Amethyst, 155 (1997)

Blue, 2,800 (2004)

Marigold, 5 (1999)

Smoke, 40 (2010), 55, 200 (both 2011),
   65, 70 (2012), 50 (2013), 45, 65 (both 2016)

Shell and Sand plates, 9 inches

Green, 75 (2006), 135 (2007),
   75, 95 (both 2012)

Helios, 45 (2010)

Marigold, 800 (2003), 950, 2,000 (both 2006),
   750 (2012), 450, 2,100 (both 2014),
   425 (2016)

Purple, 425 (2011), 700 (2012),
   900 (2013), 1,300 (2014), 475, 1,800 (both 2016)

Purple, electric, 5,750 (2008),
   2,300, 2,400 (both 2011), 2,600 (2012)

Smoke, 1,000 (2008), 1,500 (2009),
   700, 750 (both 2011), 575, 750 (both 2012),
   850 (2016)

Teal, 75 (2006), 85 (2007)

Shell and Sand bowls, 7-8 inches

Green, 35, 45 (both 2014), 20, 55 (both 2015)

Green, in metal frame, 170 (2006)

Helios, 20 (2005)

Lavender, 325 (2004), 215 (2006), 130 (2016)

Lime green, 35 (2016)

Marigold, 15 (2010), 35, 50 (both 2011),
   20 (2012), 80 (2013), 10 (2016)

Marigold, pumpkin, 350 (2003)

Purple, 60, 110, 195, 285 (all 2013),
   100, 110, 175, 250 (all 2014),
   85, 150, 155, 165, 325 (all 2015),
   65, 100, 135, 350 (all 2016)

Smoke, 225 (2004), 95 (2011), 185 (2014),
   140 (2016)

Teal, 225 (2004)

Updated 1/22/2017