I'm going to try a really high mag macro shot of the scutellum, if you wouldn't mind looking it over one more time! Thanks a lot for the help, I'm clueless with Clicks! You might like the Melanotus pic I posted too. The genitalia are exposed, so a species might be possible on that one!

That looks really clear to me! I would say Cardiophorinae for sure. The other diagnostic would be the prosternum intercoxal process short and obtuse (not a very good click mechanism). I think your mounting triangle may obscure this. The rounded pronotum and incised scutellum are rather diagnostic, so I recognized it at a glance. However, the genus is not really obvious. Horistonotus and Cardiophorus are similar, but are differentiated by a pronotal carina on the lateral margin: fine and long extending onto the hypomeron in Cardiophorus. Also if the tarsi have membranous lobes, then it might be Esthesopus (it looked like no lobes to me, but the tarsi are not perfectly in focus.) If claws are pectinate would be Aptopus but the scutellum would look different.

The tarsi look simple and the claw is simple. However, I just looked at my specimens of Cardiophorus from all over the US: MA, OR, CA, NV, MT, NM etc, representing many species not identified except for the MA ones using Downie and Arnett. They all have a carina or groove which starts at the hind angle and run up for different lengths on the hypomeron surface. In your photo, this area is too blurry to distinguish this, I think. On my specimens this is always quite thin but easy to see at a glance. If you don't see this, you may have Horistonotus.

I'm still learning my morphology, and I can't determine what you mean. Am I looking at the dorsal shot, the ventral shot? Is it the pronotal hypomeron or the elytral hypomeron. If you mean the edge of the lower part of the pronotum on the ventral shot, then I think it might be covered by the point. If not, I can shoot that part again. If it is, I can shoot photos of a second, unpinned specimen and post them separately. Thanks a lot for your help!

Well if I were better at photography, I would just photo one of mine and you'd see it instantly. I'm still using film for my microscope photography. My digital for the microscope is pretty low pixel density.
If you look at the pronotum in side view, or tilted slightly to show the underside of the pronotum, you should see either, 1) Cardiophorus-a thin groove or carina that curves over the flat face of the pronotal hypomeron. This should extend at least 1/3 of the way toward the front of the thorax but may extend even 3/4 of the way to the front. 2) Horistonotus- No carina or groove at all OR many have a thin carina that goes straight along the side of the pronotum from base to more than half way to apex, looking kind of like a normal margin, no curving down under (onto the pronotal hypomeron). I'm working on a photo with my digital, but you should get an answer before I work this miracle. My slow response is because I just got back from three weeks in Patagonia and have >400 beetles I'm trying to process.

OK I tried my very low res picture of a Lassen Cardiophorus. I didn't know how to put it here so I added it as an ID request. You can hopefully see the line extending from the hind angle down onto the hypomeron.

I see what you're talking about. Okay, I'm gonna get that shot right away

Yes. From your last photo I agree that there is a carina. I would call it Cardiophorus for sure.

Thank you so much for taking the time to help me with this. You've really helped me with the kinds of things to look for for ID's and helped with some of the morphology that I need to learn!

I'd love to see some photos of those beetles, what groups did you collect?

I specialize in various bark beetles and was disappointed at the paucity of these on the southern beeches. However, there were really nice Tenebs running around. Also Meloe with violet abdomens and lots of Carabidae. I am trying to photograph more in habitat so I have some field pictures that are OK. I also found several male Cerambycids, Prioninae which had apparently been caught by flickers and dropped to the ground missing abdomen but still kicking- they had strongly pectinate antenna which is new for me. It should be a lot of work to ID this material. I'll send you some photos when I get them worked up.