This Sullivant illustration dates back to 1898. It is one of his earlier pieces, even though he was already in his mid 40s. Sullivant was somewhat of a late bloomer when it comes to finding a personal style (but what a style it was!) Cartoony, but very much anatomically correct.
I found this wonderful image online. It represents Sullivant's art in a polished and refined stage.
Can't take my eyes off his animals, they are the best!