I am going with Species 10-C being the Kelvans until I am proven wrong, which likely will be tomorrow (Thursday). In the TOS episode, "By Any Other Name," Spock described the Kelvans in their natural form as immense beings with hundreds of tentacles. The fossilized giant floating jellyfish in last week's episode would fit that description. And if those were the babies, who knows what they grow into. I know Michelle Paradise said the 10-C is unlike any species we’ve seen before," but we haven't seen the Kelvans in their natural form. My secondary theory is that the fossilized remains we saw last week were of the Kelvans and that they were destroyed by Species 10-C which is, in fact, a wholly new species never before encountered in any form.