The term "sawback" fits this taxon well.
The 2 subspecies of the black-knobbed map turtle, Graptemys nigrinoda, are often referred to as the southern (or Delta) black-knob, G. n. delticola, and the northern black-knob, G. n. nigrinoda. The 2 subspecies are very similar, with the southern form being darker and having a larger plastral figure than, its better known northern cousin.

There has long been controversy regarding the validity of the Delta map as a subspecies. Some contend that it is just a clinal/environmental variation of the black-knob. Because the 2 subspecies intergrade over a large part of the range, many authorities do not even attempt to define the Delta form. But since Peter Lindeman, “Dr. Map Turtle”, saw fit to recognize the southern form in his The Map Turtle and Sawback Atlas, it was high on Jake’s and my “wannasee” list. By the way, both of these subspecies are “sawbacked” forms.

At our next stop it was the lesser known, the southern subspecies, that we hoped to see.

By the time we reached our destination a few miles north of Mobile, AL the parking lot was filled, there were dozens of boats containing happy fishermen on the oxbow, and Delta map turtles (adult size, 3-8”, females the larger) were on every exposed snag. When Jake realized that the turtles were so used to the fishermen that in very un-map- turtle-like manner they continued basking unless closely approached, he shook his head in disbelief.

It was a picture taking kind of morning!

Adult female Escambia map turtles develop hypertrophied heads.
Jake and I had been talking about making a photographic foray for the various southeastern map turtles (Graptemys) for some time. One hot day in June 2016 we decided that the time had finally come. We had motored to Florida’s western panhandle in search of a bog frog (successful), and the first map turtle on Jake’s list, the Escambia map, was just a hop, skip, and jump further westward. So we found a motel and at sunrise the next morning we began our hopping, skipping, and jumping to the Escambia River drainage, where from a fishing dock, Jake got his lifer Escambia map turtle, G. ernsti, an adult female. The turtle was well above the waterline on a distant snag, but was immediately identifiable.

Although this broad-headed map turtle had been known to exist in the Escambia, Yellow, and Shoal rivers system for years, until 1992 it had been considered the easternmost population of the Alabama map turtle. Females attain a length of 7 to 10+ inches. Males are about half that size.