CRUSTACEA. LEPTOSTRACA 321 



resemble the Phyllopoda. But they differ from them in 

 the clear separation of the thorax from the abdomen ; in 

 the possession of a rostrum and a mandibular palp ; and 

 in the long anterior antennae. Stalked eyes are found in 

 some Phyllopoda and in many Malacostraca. 



The Leptostraca are clearly generalised types, and are 

 probably to be regarded as the last survivors of a primitive 

 group of Crustacea. The Order is, however, without 

 representatives in post-Triassic rocks ; but a number of 

 Crustacea which closely resemble the living Leptostraca 

 in the form of the body, with in some cases a movable 

 rostrum, are found in the Palaeozoic formations; they 

 differ, however, in being much larger, and, usually, in the 

 caudal fork consisting of more than two spine-like pro- 

 cesses. The appendages of these Palaeozoic forms are 

 almost unknown, and consequently it is difficult to deter- 

 mine their affinities satisfactorily. Masticatory organs in 

 the stomach are stated to occur in some of the fossil 

 forms. Some of the principal Palaeozoic genera are 

 described below. 



Hymenocaris (fig. 139). Carapace semi-oval, smooth, not 

 bivalved. Abdomen formed of eight 

 segments, and with four, five, or six 

 caudal spines. Lingula Flags. Ex. H. 

 vermicauda. 



Ceratiocaris. Carapace bi- 

 valved, often marked with striae, oval, 



narrow in front, truncated behind and _. „„_ TT 



. .big. 139. Hymenocaris vermi- 



with a lanceolate rostrum in front. cauda, Lingula Flags. x£. 



Thorax and abdomen formed of four- 

 teen or more segments, the first seven or more being covered by the 

 carapace ; telson long and pointed, with two lateral spines. Tre- 

 madoc Beds to Carboniferous. Ex. C. stygia, C. papilio, Ludlow 

 Beds. 



w. p. 21 



