Seashell #17 is a classic clamshell. I modified the second one to be bolder and brighter.
Seashell #18 is one of my favorites, kind of a broken shell that washed up on the beach. I love the being able to see the cross section. I enhanced the color on the lower image.
Seashell #19 is a standard shell that appears to be striped. I enhanced the second image by adding more blue and purples and some pebbly texture.