green river fish fossil Knightia?

[blackmoth]

16cm long,  looks like some kind of Nightia nut I am not sure.

[Huntonia]

Looks more like a diplomystus to me but it's harder to tell when they're preserved like this. 

[blackmoth]

1 hour ago, Huntonia said:

Looks more like a diplomystus to me but it's harder to tell when they're preserved like this. 

How to tell the difference between a Knightia and a Diplomytis, particulaly when the jaw is not so clear and distinctive?

[Huntonia]

6 hours ago, blackmoth said:

How to tell the difference between a Knightia and a Diplomytis, particulaly when the jaw is not so clear and distinctive?

There is some difference in the shaping of the head and some other features, but the best way to tell is the distinct anal fin on diplomystus. Here's an image I found on Google so you can see better. 

 

 

Unfortunately your specimen is very worn and whatever anal fin there might have been appears to be missing. 

[Fossildude19]

I'm going to disagree here, and say this is a Knightia eocaena. 

The placement of the pectoral fins, the lack of a humped back, and the subtle forking of the caudal fin all point to a Knightia, rather than Diiplomystus. 

I was unsure at first, and thought it might be a Diplomystus as well, but the differences are there.

 

[Randyw]

Here’s a picture of one of my Diplomystus for comparison. The jaw, pectoral fin, hump back, robust gill cover plate, and anal fins are easiest diagnostic features for diplomystus. So I also believe yours is a knightia

[Huntonia]

Taking a closer look and reading Tim and Randy's rationales, Knightia does seem more likely. 

[-Andy-]

Another vote from me for Knightia

[blackmoth]

I noticed one thing standing out is the hollow cylindrical bone piece behind the orbital cavity, what is it?

[Fossildude19]

Probably an out of place vertebra.