It is probable that muticus and calvatus would be capable of interbreeding if they were not spatially isolated.
"Description of a New Softshell Turtle From the Southeastern United States"
Robert G. Webb
Briefly, they look something like Jews, practise a very debased form of the Jewish religion, are civilized and clever after a fashion, but in the last stage of decadence from interbreeding-about nine thousand men is their total fighting force, although three or four generations ago they had twenty thousand-and live in hourly terror of extermination by the surrounding Fung, who hold them in hereditary hate as the possessors of the wonderful mountain fortress that once belonged to their forefathers."
"Queen Sheba's Ring"
H. Rider Haggard
There is no variation of the social monotony, and the result is socially the same as close consanguineal interbreeding.
"Consanguineous Marriages in the American Population"
George B. Louis Arner