CRUSTACEA. 159 



and comprises small Bivalved Crustaceans, with " dark, horny, 

 sub-quadrate valves, obliquely ridged from umbo to angles, 

 and ornamented with distinct lines of growth parallel with the 

 border" (Jones). Leaia is a very widely distributed genus, 

 but all the known species belong to either the Carboniferous 

 or Permian Rocks. 



ORDER PHYLLOPODA. 



Crustacea, mostly of small size, the carapace protecting the head 

 and thorax, or the body entirely naked. Feet numerous, never less 

 than eight pairs, mostly foliaceous or leaf like, branchial in function. 



Most of the living Phyllopods are inhabitants of fresh waters, 

 but some live in the sea (Nebalia\ and others affect waters 

 which are abnormally salt (Artemia). The two most interest- 

 ing recent forms, as bearing on fossil examples of the order, 

 are Limnadia and Apus, both of which live in fresh water. In 

 Limnadia the body is enclosed in an oval bivalve carapace, 

 and there are from eighteen to thirty pairs of membranous 

 leaf-like feet. In Apus the carapace is clypeiform, and protects 

 a considerable portion of the abdomen ; and there are sixty 

 pairs of feet, of which all but the first pair are foliaceous. 



Leaving out of sight the genera Estheria and Leaia, the 

 Phyllopods are almost exclusively palaeozoic in their distribu- 

 tion, and are chiefly, though not exclusively, known by their 

 carapace- valves. The best known genera are the Hymenocaris 

 of the Lingula flags, the Caryocaris of the Skiddaw slates, the 

 Peltocaris and Discinocaris of the Lower Silurian, the Ceratio- 

 caris of the Upper Silurian, and the Dithyrocaris of the Car- 

 boniferous Limestone. These forms have a general resem- 

 blance to one another, and are believed to be most nearly allied 

 to the recent Apus, whilst they are exclusively palaeozoic. The 

 genus Aspidocaris, however, is allied to the preceding, and is 

 found in the Triassic period. 



In Hymenocaris (fig. 102, b} the carapace is comparatively 

 large, sub-triangular, apparently not bivalved ; there are nine 

 free abdominal segments, and the last carries three pairs of 

 unequal lanceolate appendages. In Caryocaris the carapace is 

 bivalved, pod-shaped, and truncated behind, and the last ab- 

 dominal segment carries three spines. In Peltocaris and Dis- 

 cinocaris (fig. 102, c.} the carapace is rounded, with concentric 

 lines of growth, a dorsal furrow being present in the former, 

 but wanting in the latter. In both there is commonly a wedge- 

 shaped indentation in front, caused by the separation from the 

 carapace of the anterior portion of the head. In Gtratiocaris, 



