Two sorts of ships, a "winged" and a fairly alien "dendril" type of ships. Each sheet has 150 ships. The preview is shrunk at 50%, the sheets are at a higher resolutioon.

These ships came into existence as a byproduct of a shooter concept study that I once made. Example screens are shown at the right side of the preview. The dendril type ships can certainly be used in a space invaders type of game as well.

The sprite sheets come with full transparency and are ready to use.

More ships: http://opengameart.org/content/shell-type-ships

Mines: http://opengameart.org/content/mines-aka-space-urchins