Quasillites is a fossil genus of ostracod from the Devonian and Carboniferous Periods.

[2] The surface of each valve is covered in longitudinal, bifurcating ridges, which curve and resemble a finger print.

[2][3] The ribs and spines on the carapace are in front of the posterior margin, and are similar to those of other ostracods such as Bufina, Parabufina, and Healdia.

[4] Devonian examples of Quasillites, such as Q. lobatus, Q. obliquus, Q. subobliquus, and Q. angulatus, are known from shales and claystones in northern New York, Eastern Ohio, central Pennsylvania.

[7] This genus has also been found in the Lower Carboniferous edge of the Illinois Basin; in central Indiana.