CRUSTACEA. PHYLLOCARIDA. 191 



Hymenocaris. Carapace semi-oval, smooth, not bi- 

 valved. Abdomen formed of eight segments and four or 

 five caudal spines. Appendages unknown. Lingula Flags. 



Ceratiocaris. Carapace bivalved, often marked with 

 striae, oval, truncated behind and with a lanceolate rostrum 

 in front. Body formed of fourteen or more segments, the 

 first six or seven being covered by the carapace ; the last 

 segment is produced into a strong spine and provided 

 with two smaller spines. Ordovician to Carboniferous. 



Distribution of the Leptostraca. 



The three living genera are all marine. The earliest 

 representative is Ceratiocaris which occurs in the Tre- 

 macloc Beds and ranges on to the Devonian. Hymenocaris 

 is found in the Lingula Flags; Echinocaris in the De- 

 vonian; and Dithyrocaris in the Carboniferous. 



ORDER. THORACOSTRACA. 



In this order there is a cephalothorax formed by the 

 fusion of the head with some or all of the segments of the 

 thorax, and this is covered dorsally by a shield or carapace. 

 There are thirteen segments in the cephalothorax, six 

 in the abdomen and a terminal telson. The eyes are 

 compound and are usually placed on movable stalks. 

 The Thoracostraca includes four sub-orders, (1) Cumacea, 

 (2) Stomatopoda, (3) Schizopoda, (4) Decapoda. The 

 first and third are unknown in the fossil state ; and only a 

 few forms of the Stomatopoda have been found. 



SUB-ORDER. DECAPODA. 



The Decapods include the lobsters, crayfishes, and 

 the crabs. The head and in almost all cases all the 

 segments of the thorax are covered by the large and 



