[camillomiller]

Not really. And it's not a translation error. Here's the original:

"Velocissimum omnium animalium, non solum marinorum, est delphinus, ocior volucre, acrior telo, ac nisi multum infra rostrum os illi foret medio paene in ventre, nullus piscium celeritatem eius evaderet."

Looking at pictures of dolphins, I guess he's referring to the fact that they have a very long beak ("rostrum") and therefore the mouth cavity ("os") is tucked in a lot more than other fish-like creatures. So much, indeed, that it might look like its mouth orifice is almost close to its belly.

He could have explained that a bit better, I guess, but something might be lost in the different nuances of meanings we give to "muzzle" and "mouth". Let's cut him some slack, though. The guy was able to identify a singular dolphin thanks to the cuts made (by sailors, I guess) on its dorsal fin. That's a technique researchers are basically still using today in the photo-identification of sea mammals.

[steamer25]

Many Pliny was conflating dolphins and sharks?

E.g., "his mouth is situated much below his muzzle, almost, indeed, in the middle of the belly" could be reasonably applied to a picture like this one: http://www.sharkwallpapers.net/bulkupload/wallpapers/Great%2...

Maybe there are other animals mixed in there too. This seemed odd to me (although I'm far from a marine biologist): "...would present his back for him to mount, taking care to conceal the spiny projection of his fins in their sheath..."