My dog brought home a twi skulls that I can't identify.

Factual Questions
1

Can you identify these skulls that my dog brought home? These were found on (or very near) a Cape Cod seashore. I can’t even tell if they’re mammal, reptile, or fish.
http://www.bizer.com/140830_221523_0733A.jpg
The top of the face is very rough, like sandpaper. At first, I thought sand had fused to the skull, but I soaked them in water overnight and none of the “sand” came off.

The skulls are lying on a glass table. The skull on the right is face-down. In both cases, the lower jaw was not found.
Here’s a larger version of the same photograph. http://www.bizer.com/140830_221523_0733F.jpg

2

Fish skulls, I think. Not sure what species, but for general comparison, here’s the skull of a cuckoo gurnard:

3

OK, I think it is actually a gurnard species that is local to you called the Northern Sea Robin - there’s a picture of a very similar specimen about halfway down the page here.