DEFINITION OF THE 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 maxillae 

 are foUaceous ; 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 Umbs. The 

 hindmost two pairs of thoracic appendages are ambu- 

 latory limbs, hke 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 

 podobranchise, but the first of these is always more or 

 less completely reduced to an epipodite. More or fewer 

 arthrobranchise always exist, Pleurobranchiee may be 

 present or absent. 



In this tribe of Astacina there are two families, tlio 

 Potamobiidce and the Parastacidce ; and the definition of 

 each of these families is formed by superadding to the 



