Watermelon and Citron (Citrullus lanatus)

Todays hybridized watermelons offer a thinner rind and fewer seeds than heirloom types our grandparents grew. The heirloom types are just as sweet and flavorful (if not more so) as todays hybrids. All watermelon are outbreeding, and will readily cross with any variety of watermelon, and Citron (a fruit grown for preserves and candied fruits, not common to the US home gardener) 1/2 mile isolation distance is recommended for seed purity. Hand pollination techniques are the same as for cucumber and squash.
When a watermelon is ready to eat, the seeds are mature, and viable. To collect seeds, simply eat the melon, but spit the seeds into a cup. Wash the seeds with mild dish washing soap, and rinse thoroughly. Seeds will remain viable for 6 years when stored in a cool dark, dry place (refrigerator).

Seed savers exchange offers 94 varieties, and the Garden Seed Inventory lists sources for 148 varieties. There are 315 seeds per ounce, or 11 per gram.

What watermelon varieties would be considered heirloom?

I believe that any in Seed Savers Exchange is heirloom. They only sell heirloom seeds.

I deal mainly with Victory Seeds company...

http://www.victoryseeds.com/

here's why:

The Safe Seed Pledge

"Agriculture and seeds provide the basis upon which our lives depend. We must protect this foundations as a safe and genetically stable source for future generations. For the benefit of all farmers, gardeners and consumers who want an alternative, we pledge that we do not knowingly buy or sell genetically engineered seeds or plants. The mechanical transfer of genetic material outside of natural reproductive methods and between genera, families or kingdoms, poses great biological risks as well as economic, political, and cultural threats. We feel that genetically engineered varieties have been insufficiently tested prior to public release. More research and testing is necessary to further assess the potential risks of genetically engineered seeds. Further, we wish to support agricultural progress that leads to healthier soils, genetically diverse agricultural ecosystems and ultimately people and communities."

Here is their watermelon selection, all are heirloom types

http://www.victoryseeds.com/watermelons.html