Identifying a Greater Siren vs. A lesser Siren

[RandallRock]

I purchased a pair of sirens that were mislabeled as "Congo Eels" or the amphiuma and I have had the animals for 5 months now and have tried to research the subject and I just can't tell which they are. They're about 16-18 inches long and the fingers tips have already turned black which I'm assuming indicates they're sexually mature like the many Axolotls I have. I have the two in a 55-gallon and they've grown maybe two or three inches since I've purchased them and allowed them to regenerate tail-ends and an arm after being crammed in a 20 long for however long and no doubt biting each other senseless.
they look like this which is a picture of a greater, however they haven't broke 2 feet yet and are sexually mature so I'm confused. Is there any absolute definite way to tell at a glance? And further more is there any clues to gender as well? Thanks

http://www.amphibiainfo.com/gallery/caudata/sirenidae/siren/lacertina/siren_lacertina_abs.jpg