An ancient genus of mostly submerged aquatic plants of soft water lakes that often look very similar, with identification largely based on the size and ornamentation pattern of mature megaspores. With ripe megaspores our two common species are easy to distinguish with a 10× hand lens. Quillworts typically grow with other, similar rosette forming aquatic plants characteristic of soft water lakes, ponds, and streams with sterile, sandy or gravely substrates such as Eriocaulon, Lobelia dortmanna, Juncus pelocarpus, Sagittaria, etc.

1. Megaspores ± densely ornamented with spines and tubercles that all have separate bases, not fused into ridges or a network.

I. echinospora

1. Megaspores ornamented with tall, elongated ridges, forming at least a loosely connected pattern.

2. Megaspores large (> 0.6 mm) with the irregular ridges, stopping and starting and not formed into a uniform network and with a narrow band of tall, coarse spines about the equatorial ridge.

I. lacustris

2. Megaspores small (< 0.5 mm) with the ridges formed into a uniform closed network (honeycomb pattern) and with no distinctive equatorial band.

I. engelmannii