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Ugh! I remember this book too. It had a section on Dale Russell's Dinosauroid. Want the title too.
Could this be The National Geographic Picture Atlas of Our Universe? It definitely had a section on hypothetical extraterrestrial life, the relevant pages scanned and reposted here
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Cool, but no.
Found it! Extraterrestrials: A Field Guide for Earthlings, by Terence Dickinson and Adolf Schaller.
Damn it! I missed my chance of showing my obscure knowledge! I actually have this book.
This is a book I read as a kid about the possibilities of extraterrestrial life. One section of the book had pictures and descriptions for various hypothetical other planets, like a low-gravity one, a high gravity one, a jungle one, an ice one, and a gas planet where life floated around in the atmosphere. The illustration from the latter was used for the cover, showing a whale/manta ray type creature on a bluish background.