A FOSSIL'S KITH AND KIN 199 



CHAPTER XIV 



TRIBE IV. — ASTACIDEA 



The first antennae carry two multiarticnlate flagella ; the 

 second are furnished with a scale. The trunk-legs have 

 seven distinct joints. The first three pairs of trunk-legs, 

 and sometimes the other two pairs also, are chelate. The 

 first pair are the largest. The branchia? are well developed. 

 The first segment of the pleon has appendages, except in 

 the Parastacida3. 



In this tribe the young are said to be hatched in the 

 Megalopa form. It contams four families — the Eryontida), 

 Nephropsidse, Potamobiidas, and Parastacid^e. 



Familif 1. — Eryontidce. 



The carapace is dorsally depressed, with little or no 

 rostrum. The eyes are wanting or abnormal. The second 

 antennae have a long multiarticulate flagellum. The third 

 maxillipeds are pediform. The pleopods, excej.t the first 

 pair, have a process attached to the inner branch (the 

 stylamhlys of Spence Bate's terminology). The uropods 

 have no transverse suture. The telson is tapering. 



To this family are assigned seven genera, but one of 

 these, Eryon, Desmarest, 1820, which gives its name to 

 the family, is a fossil genus from the Lias of England and 

 the lithographic limestone of Bavaria. It is only in re- 

 cent years that the depths of the ocean have yielded forms 

 which appear to be properly classified in close proximity 

 to the ancient fossil species. 



Polycheles, Heller, 1863, has the anterior angles of the 

 16 



