The genus Æger of Münster was established in 1839, to contain some of the most beautiful forms of Prawn-like Crustacea found in the Solenhofen Limestone of Bavaria (see Beiträge, vol. ii. p. 64). Dr. Oppel, in his Palæontologische Mittheilungen (Stuttgart, 1862), p. 109, thus defines the genus:

The inner antennæ (antennules), with their long bifid filaments, start from three strong basal articulations, and attain in most specimens to twice the length of the whole body. The antennal scales are thin and very long. The basal joints of the inner antennæ are finely serrated along their border. They are much elongated, and project further in front of the head than the outer antennæ, as is the case in the existing Shrimps.