Abstract
A new species of coccidian (Protista: Apicomplexa: Eimeriidae) collected from the faeces of a midland brown snake Storeria dekayi wrightorum Trapido (Ophidia: Colubridae) in Arkansas, USA, is described. Oöcysts of Isospora holbrooki n. sp. are subspherical to ovoidal with a smooth, colourless, bi-layered wall, measure on average 27.1 × 24.0 µm, and have a length/width (L/W) ratio of 1.1; both micropyle and oöcyst residuum are absent, but a polar granule is present. Sporocysts are ovoidal, 14.8 × 10.0 µm on average (L/W 1.5); the Stieda body is nipple-like, the sub-Stieda body is ellipsoidal and the sporocyst residuum is composed of coarse granules in a cluster. Sporozoites have a spheroidal anterior refractile body, a subspheroidal posterior refractile body, and one centrally-located nucleus. This is the first description of an isosporan from the snake genus Storeria Baird & Girard as well as the largest oöcysts and sporocysts of any previous snake isosporan to date.