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!