DEFINITION OF TITE GROUP ASTACINA, 255 
partially free, are united into a cephalothorax, the tergal 
wall of which has the form of a continuous carapace. 
The carapace is produced in front into a rostrum, at the 
sides into branchiostegites. 
The eyes are placed at the ends of movable stalks. 
The antennules are terminated by two filaments. The 
exopodite of the antenna has the form of a mobile scale. 
The mandible has a palp. The first and second maxille 
are foliaceous; the second being provided with a large 
scaphognathite. There are three pairs of maxillipedes, 
and the endopodites of the third pair are narrow and 
elongated. The next pair of thoracic appendages is much 
larger than the rest, and is chelate, as are the two fol- 
lowing pairs, which are slender ambulatory limbs. The 
hindmost two pairs of thoracic appendages are ambu- 
latory limbs, like the foregoing, but not chelate. The 
abdominal appendages are small swimmerets, except the 
sixth pair, which are very large, and have the exopodite 
divided by a transverse joint. 
All the crayfishes have a complex gastric armature. 
The seven anterior thoracic limbs are provided with 
podobranchie, but the first of these is always more or 
less completely reduced to an epipodite. More or fewer 
arthrobranchie always exist. Pleurobranchie may be 
present or absent. 
In this tribe of Astacina there are two families, the 
Potamobiide and the Parastacide ; and the definition of 
each of these families is formed by superadding to the 
