Hi Curt, thanks for looking into this. I have a short and long response (see ridiculously detailed comment - aka rant - to follow if you have time regarding the raised tubercles of D. ambiguus and also D. furvus). I think you are looking at an artifact of the photo since it is so shiny. That possible raised area is actually a slight depression. I am not sure if I have ever seen D. ambiguus and it would be nice to have a side-by-side comparison of pronotums since D. teter's should be more narrow at the base. Based on an incredibly small sample size (N=2) regarding D. furvus, I think that raised tubercle might be a very subtle thing (as discussed in comment/rant below). D. ambiguus is not included in Bousquet's NE Carabid key as it has an overall more southern distribution than D. teter. However, Bousquet's NA catalog does list D. ambiguus from PA. Still, it is hard for me to believe a beetle at the very northern tip of the Allegheny National Forest would not also be in Southern Canada. All of that supports D. teter, but not with 100% certainty.

D. teter has pronotum with lateral borders slightly, but distinctively, convergent toward hind angles AND the anterior angles are only moderately projected forward. I notice that the width across head + eyes is about the same as width of pronotal base. I agree with Curt that the imaged pronotum appears a bit wider than I would like for D. teter, but I blame that on photographic trickery. D. ambiguus and D. furvus have pronota with sides divergent whose hind angles easily reach the humeral angles of the elytra AND the anterior angles are strongly, abruptly projected. The last two species also have width of head + eyes much narrower than pronotal base. The statement by Bousquet (2010: 120) is misleading at least with regard to male D. teter and male D. politus. Some of my specimens in each case have more than than a total of two anal setae. In particular, I have a male D. teter with 2 left and 3 right anal setae. Also the left mandibular "shine" in the image does not exactly correspond to where the tubercle would be expected. Frank, I'll take a look at your long version below when I find time.

Thanks Peter for your opinion. I will send you one of the three males after the holidays so you can look at the actual specimen.

So here is my long answer....
I have been paying more attention to my Dicaelus lately. A few months ago I noticed the beetles I was 100% sure were D. elongatus all had a tubercle on their left mandible. This was driving me crazy since I knew that couldn't be right. I found George Ball's original book Ball (1959) and must say it is a beautiful piece of work. He spends a lot of time discussing the mandibular structures of the Licinini. So, I relaxed and spread the mandibles on all of my Dicaelus in order to take a better look. This amounted to various forms of torture for me and the dried specimens. But, in the end, I was successful apart from one left D. elongatus mandible that snapped off and went flying across the room - incredibly I found it, now glued to a point under the label (also not counting an entirely wrenched off head from another D. elongatus since I was able to glue it back on - I swear you can't even tell what happened!). I was correct about all my D. elongatus identifications - what I mistook as a tubercle was actually a tooth (big difference in the key). I can't say anything for sure about D. ambiguus since I don't think that I have one in my collection. But, I do have two D. furvus, and in those specimens, the "tubercle" is not an obvious character at all (to me, "tubercle" is something akin to the pointy bump on a female rhinoceros beetle's head). According to Ball, the left "type IIIb" mandible is a complicated structure (see page 101). The base ("retinaculm") is thick in the dorsal-ventral plane. The terebrum is long and slender. It twists as it tapers and looks a bit like a narrow propeller blade. There is a convergence of edges just anterior to the labrum that gives, but only at certain angles (in my opinion), the appearance of a slightly raised surface. This is very different from the left "type Ia" mandible of D. elongatus that, in dorsal view, is basically a flat triangle with a medial tooth (see pages 99-100). Ball's key mentions a "dorsal ridge or tubercle", but his species description is perhaps more nuanced: "dorsal surface with low protuberance near the the proximal junction of the terebral margin and retinacular ridge". His actual drawing of the mandible (Plate VII, Figure 86) does not show a tubercle. So, I am not sure if Cieglar's figure 54 is entirely accurate? At a minimum, it might over-emphasize that structure? I think the best way the separate D. elongatus and D. furvus is by overall shape and also by the presence of three vs. two punctures with setae on the lateral pronotal margin.

Thanks for all the detailed observations! The dorsal lump/tubercle/tooth/projection on the left mandible of D. furvus has been easy to see (with a microscope!) on all the specimens I've seen, and to my eye looks very different from the mandible of D. elongatus. The structure is more evident in D. ambiguus (maybe just because the beetles are larger). I will send you a D. ambiguus soon for you to check out.