948 PRINCIPLES OF STRATIGRAPHY 



developed, forming the corona. The Phyllocarida were of great 

 importance in the Palaeozoic. They generally had head and thorax 

 enclosed in a carapace consisting mainly of two valves, with acces- 

 sory pieces. The ringed abdomen and the tailpiece or telson (often 

 triple) projected beyond the carapace. 



The Decapoda have head and thorax united into a cephalo- 

 thorax, and covered by a single carapace, or with one segment 

 free. Each of the thirteen cephalothoracic segments has a pair of 

 jointed appendages, some of which are modified into antennse or 

 month-parts. The abdomen consists of seven segments, the terminal 

 one being a telson. In the Macrura (lobsters, crayfish, etc.) these 

 segments. are all visible, but in the Brachiura (crabs) they are gen- 

 erally turned under the carapace. Locomotor appendages (pereio- 

 poda) are in five pairs, and with few exceptions each consists 

 of seven joints. Some of the final joints are claw-like, others 

 paddle-like, and others again merely pointed for walking pur- 

 poses. Six pairs of abdominal legs occur. The claws are often 

 found fossilized separately. Decapods first appear in the Triassic. 

 The remaining orders show various modifications of the decapod 

 type. They are mostly rare as fossils. 



Among the Accrata the Merostomata are in many respects of 

 greatest interest. Some Eurypterida in the Devonic reached a length 

 of six feet, but were smaller in other horizons. The Limulava are 

 known only from the Middle Cambric. They combined trilobite 

 with eurypterid characters. The eurypterids had a short cephalo- 

 thorax, a ringed abdomen and a telson, the body, as in Crustacea, 

 being covered with a chitinous exoskeleton, which was repeatedly 

 shed. A pair of compound eyes and a pair of median simple eyes 

 or ocelli formed the chief dorsal features of the carapace. Ventrally 

 this bore six pairs of jointed appendages, the first preoral and che- 

 late, the others non-chelate, the last usually forming a large paddle. 

 The first six segments of the abdomen bore broad, leaf-like append- 

 ages, referable to "gills." The posterior segment and telson were 

 without appendages. The number of known species is over 150. 

 The Synxiphosura (Cambric to Siluric, few species) had a trilo- 

 bitiform abdomen, which, in the adult Xiphosura, of which Limu- 

 lus, the horseshoe crab, is the only living example, was fused into a 

 single piece, though still indicating the segments and trilobation. 

 The fossil species (few in number) have been obtained from the 

 Upper Devonic, the Carbonic, and (genus Limulus, only) from 

 the Mesozoic and Tertiary of Europe. 



The scorpions and spiders are more complex Acerata adapted 



