Defiantly not a Narrow-Headed Garter though the pics do at first glance have a superficial resemblance to them. When held in the hand and A/B'd with Narrow-Headed Garters, a species I am very well acquainted with, there is little to no similarity. We have also ruled out any of the new world Water Snakes. This animal has been narrowed down to a European Natrix but which of 2 or 3 species is uncertain. Possibly a hybrid of 2 such Natrix.

Regardless it is an interesting snake and quite the enjoyable captive. It thrives well with very basic husbandry. It puts up a very aggressive show, hisses loudly, but never bites or skips a meal. I am glad I acquired it even though it was not intentional. It is a keeper.
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Randy Hallman
Firehouse Herps