Latreille, 1802
Body rather cylindrical or oval. Rostrum well-developed. Epistome free from carapace. Anterior three pereiopods with true chelae (few subchelate in pereiopods II and III) and with first pair of pereiopod largest and most robust. Fourth pereiopod, and usually also the fifth, without true chelae. Tail-fan entirely calcified (marine species) or more than half of telson calcified (freshwater species). Posterior margin of telson always broadly convex.
This group includes the true lobsters and crayfishes. The Astacidea can be easily distinguished from the other lobsters by the presence of chelae (pincers) on the first three pairs of legs, and by the fact that the first pair is by far the largest and most robust. The last two pairs of legs end in a simple dactylus, except in Thaumastocheles, where the 5th leg may bear a minute pincer.
The infraorder consists of three superfamilies, two of these, the Astacoidea Latreille, 1802 (crayfishes of the northern Hemisphere) and the Parastacoidea (crayfishes of the southern Hemisphere), include only freshwater species and are not further considered here. The third superfamily, Nephropoidea, comprises the true lobsters, treated below.