I found this on the beach in St. Augustine, Florida. The sand grains should give you an idea of size. What is it?

I photographed this unusual tree in a park in Jacksonville, Florida. The orange things are ripe fruit, and the green ones are .....green fruit. What is this tree, and why is considered to be an undesirable, invasive species?

The fossil shells in this photo look much like clams or oysters, but they are remains of a totally different group of organisms. What are they?