As a specialist in Maya epigraphy, I am 100% sure that this is a fake. I know that art history presents some special documentation problems, and I will provide sources at the end of my post, so I hope the moderators will bear with me.

First, as to the "hieroglyphs" on this shell, there are no verbal affixes, which would give you grammatical information about a verb. These are very easily recognizable in real Classic Mayan texts, and I would absolutely expect to see one or two here if this were a real sentence in Classic Mayan. Their absence is a very serious objection to the authenticity of this piece. To play the Devil's advocate for a moment, I could point out that technically you don't need verbal affixation to write a meaningful sentence in Classic Mayan, since a peculiarity of the language is that nouns are written, and sound in the spoken language, exactly like third-person stative verbal constructions which can constitute complete sentences in their own right. For example, bahlam can mean "jaguar" or "it is a jaguar" - linguists would render the latter bahlam-0, where the zero represents a third-person singular absolutive pronoun that exists conceptually but is not written or pronounced. So why isn't this just a set of four stative verbal expressions? I have never come across such lists in real Classic Maya inscriptions, at least not outside of some larger narrative context. Conceivably it could happen, but, well, it just doesn't.

The second problem is even more serious. None of the signs in the text are attested anywhere else in the corpus of genuine Maya hieroglyphic inscriptions. While /u/snickeringshadow tentatively identified some elements that looked to him/her like real syllabic signs, I can confirm, having read thousands of real inscriptions over the past ten years, that they are not. That leaves the possibility that these are four logograms (signs for whole words), but if so, this is the one and only surviving inscription in which those signs were written. So we might just suppose that this is a list of four stative phrases about nouns ("It is an A. It is a B. It is a C. It is a D.") - but for all of those nouns to be written with totally unique word signs, never used before or afterwards in any other inscription? For none of them to be written with a single real syllabic sign? That's just implausible.

That said, this does stylistically approach real ancient Maya art in some respects. My job here wouldn't be complete if I didn't point out from which genuine pieces the forger got his or her inspiration, and you can see one of them here:

http://www.slub-dresden.de/en/collections/manuscripts/the-dresden-maya-codex/

If you look through the Dresden Codex for a while, you'll see fingers rendered in a very similar way to those on the shell - kind of gracefully outcurved, a bit like thick spaghetti. This way of depicting hands is pretty much specific to that document. The headdresses are probably inspired by depictions of the Maize God in that same document - only the Maize God's actual head sprouts into lush vegetation, whereas the people on the shell are supposed to be regular humans. The net result is headdresses that don't quite look like maize foliage, but also don't look like real Maya depictions of headgear. One of the other reasons that this piece looks so strange to me is that the way the legs are depicted doesn't match the obviously Dresden-inspired hands and headgear. That's because the Dresden Codex was produced in the eleventh or twelfth century A.D. in the northern Yucatan Peninsula, but the legs are patterned after works made hundreds of miles to the south, in the eighth century A.D. You can see one such piece here:

http://www.authenticmaya.com/images/cancuen%20panel3.jpg

This is a limestone panel made at the site of Cancuen, in the late 700s.

So, to sum up: (1) the text doesn't work grammatically, (2) the text uses glyphs that are either imaginary or totally unique, and (3) the images are a pastiche of styles and motifs from different parts of the Maya world, separated by hundreds of years. I don't know if it was mass-produced, but this shell is positively a fake. It was made by someone who had some examples of Maya art at hand (probably including drawings of the Dresden Codex, not photos of the original), but didn't really understand what he was looking at or how to put those pieces together in a convincing way.

I promised sources, and here they are! Introductory sources on Maya writing:

Coe, Michael D., and Mark Van Stone, 2005. Reading the Maya Glyphs. Second edition. Thames and Hudson, New York.

Montgomery, John. 2002. How to Read Maya Hieroglyphs. Hippocrene Books, New York.

Stone, Andrea, and Marc Zender. 2011. Reading Maya Art. Thames and Hudson, New York.

The Cancuen panel was recently published in:

Finamore, Daniel, and Stephen D. Houston, eds. 2010. The Fiery Pool: The Maya and the Mythic Sea. Peabody Essex Museum, Salem.

That's also a great source for genuine examples of ancient Maya shell art, early and late, from all over the Maya world.

I have been looking through Thompson's Rise and Fall of the Maya Civilization, Schele's The Blood of Kings, and Webster's The Fall of the Ancient Maya and I cannot find this image. Granted, these works are not in any way a definitive collection of Maya inscriptions and images, but they do present some of the best known pieces. Personally, and I am not a Mayanist so take this with a grain of salt, I think it is a mixture of images from Bonampak. They did inscribe on shells, however, and the sitting figure is similar to many other figures in that position. I would be interested to see what Mayanists think about it.

Edit It might be better to change your title to Maya Shell, since it is clearly not Nahua.

Can you take a higher resolution and large picture of the writing? I will be able to translate for you and we can get a better idea of what exactly is going on. If the writing turns out to be gibberish it may reveal that this is indeed a fake.

So I spent the last half hour trying to translate the inscription, but without success. That doesn't necessarily mean anything - it's hard and I'm not very good. But what I can tell you is that these are likely actual glyphs rather than made up symbols. Above and between the heads of the two seated figures is a glyph that looks like the verb chok which means 'to throw' or 'to scatter.' It's often used to denote scattering incense. I'd need a closer look at it to say for sure. The first glyph in the group of four appears to begin with the prefix "yo-" which is a third person possessive (his/hers/its). If I'm right about both of these, then the sentence has correct grammatical structure (Verb-Object-Subject) and the next two glyphs are probably names and/or titles. The last glyph in the set looks very similar to the name of the god "Kawil" but its not an exact match. That glyph ends with a "wi" or a "ni" sound. Again I can't tell for sure.

Anyways bottom line: I have no idea what the inscription as a whole says, but I can recognize enough elements to say that this is likely a real inscription. The art style and the glyphs both correspond to the Classic Period Maya, and shellworking was fairly common for them. I'd say this is either authentic or a high quality replica. I can't tell which just from the pictures.

EDIT: /u/cociyo actually specializes in this, and apparently I was dead wrong. Turns out it's probably a fake. Sorry.