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

Blue, smokey, 900 (2020)

Marigold, 375 (2016), 475 (2017), 725 (2020)

Smoke, 425, 450 (both 2019), 350, 425 (both 2020),
   200, 250, 425 (all 2021)

Smoke, Karen Engel auction, 425 (2017)

Smoke, marigold over, 1,400 (2013)

Shell bowls, about 7-8 inches

Amethyst, 155 (1997)

Blue, 2,800 (2004)

Marigold, 5 (1999), 5 (2018), 40, 55 (both 2021)

Marigold, spectacular, 290 (2020)

Smoke, 45 (2017), 40, 75 (2018), 90, 115 (both 2020),
   50 (2021)

Shell and Sand plates, 9 inches

Green, 75, 95 (both 2012), 95 (2018), 150 (2020)

Helios, 45 (2010), 255 (2021)

Marigold, 450, 2,100 (both 2014), 425 (2016),
   800, 850 (both 2019)

Purple, 550, 2,000 (both 2019), 750 (2020),
   195, 1,600 (both 2021)

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

Smoke, 450 (2017), 900 (2019), 195, 2,750 (both 2020)

Teal, 75 (2006), 85 (2007)

Shell and Sand bowls, 7-8 inches

Green, 20, 55 (both 2015), 10, 15 (both 2018),
   25, 80, 100 (all 2021)

Green, in metal frame, 170 (2006)

Helios, 20 (2005)

Lavender, 130 (2016), 375 (2019),
   200, 275, 500 (all 2021)

Lime green, 35 (2016)

Marigold, 55 (2019), 25, 45, 50 (all 2020),
   15, 35 (both 2021)

Marigold, pumpkin, 350 (2003)

Purple, 110, 125, 135, 180, 230, 275, 325, 525 (all 2020),
   75, 80, 90, 110, 120 (all 2021)

Smoke, 185 (2014), 140 (2016), 110 (2017), 225 (2021)

Teal, 225 (2004)

Updated 11/20/2021