Latreille, 1806
The integument of the body is very firm. The carapace is high; in dorsal view it is elongate oval in outline. In adults the carapace measures less than 1/3 of the total body length. The rostrum is narrowly triangular and short; it is depressed and its lateral margins continue for some distance on the carapace as short divergent ridges. The rostrum has no teeth. The carapace ends posteriorly in a distinct posteriorly directed median tooth that overhangs the articulation with the first abdominal somite.
The abdomen is long and narrow, more than five times as long as wide in the males, about four times as long as wide in the females. The somites are of about equal width throughout their length, they have a longitudinal carina over the base of the pleura.
The telson is about as long as the previous somite, but slightly narrower, the posterior margin is broadly rounded. The uropods are styliform. The eyes are small. The first pair of pereiopods is very strong and asymmetrical, both chelae are subchelate, the larger less conspicuously so than the smaller. The second legs are smaller, also subchelate; the other legs are simple. Epipods are present on the pereiopods.
Type species: by monotypy: Thalassina scorpionides Latreille, 1806 ( = junior subjective synonym of Cancer (Astacus) anomalus Herbst, 1804).
Until recently this genus was generally considered to have a single species, but recent investigations make it likely that more than one have to be recognized. A revision of the taxonomy of Thalassina is badly needed.