Thallium erectum is a somewhat calcified plant that is bright green when it is new and cream or greenish when it is dry. Bulbous holdfast. A compressed, rectangular to subcuneate basal segment forms the base of the thallus, from which two or more distinct segments originate in a single plane to form a somewhat folded, fan-shaped base. Large, flat to compressed branch segments that are 1 to 3 mm thick, frequently flabellate and sporadically quadrangular, with an entire, undulating, or irregularly lobed upper edge; the longest segment measures 2.2 cm in height and 2.9 cm in width.