Its definitely an echinoid (class Echinoidea). Sand dollar (order Clypeasteroida) is a reasonable guess.

Internal mold of a bivalve.

Thanks for the additional pictures. I no longer believe that is a cephalopod. That is a Gryphaea oyster, also known as devil’s toenails.

Oyster

Probably part of a broken brachiopod shell. I say that because the center raised ridge looks like the fold, which corresponds to the sulcus in the opposite valve or the corresponding shell. A scale and views of the back and sides may help confirm that.

Most likely a cephalopod such as a nautiloid or ammonoid. A scale such as a ruler and different views such as from the side is helpful.

I’m fairly confident that is a brachiopod. Do you have a locality where it was found?

I would hazard Cosmopolitodus planus (Agassiz, 1856), but a locality where this came from is always helpful.

internal mold of a bivalve or a brachiopod.

Some of the larger pieces on the bottom look like oyster shells.

Compare with an internet search for “deer hoof core”

I would say it depends on how you found it. If you found it on the ground surface, it could have been transported there through many ways: broke off an outcrop and rolled down hill, broke off and was transported by a creek or river, or it was transported there by humans. In those cases, we don’t know exactly where it came from. If you don’t know…[Read more]

Flexicalymene ouzregui Destombes, 1966

I concur with @corinne-daycross. I would go further and say this is Knightia eocaena Jordan, 1907. There are 2 species of Knightia commonly found in the Green River Formation of Wyoming. K. eocaena and K. alta. K. alta has a deeper belly.

Based on your location, it looks like the bedrock has been mapped to the Silurian Period. Orthocones, which are cephalopods, are consistent with that. You can check out this website for more information: https://silurian-reef.fieldmuseum.org/narrative/429

My first impression is orthocone as @a-trilobite suggested. Locality info is always helpful (state, nearest city or town, and the name of the pond if you have it).

@kara-freeman, @daniel-gonzalez This is a murex shell that has been weathered so that parts of it have broken off.

Not a mollusc, this is a decapod of the genus Thalassina.

Where was it found? I can’t stress enough how important the locality context is.