CRUSTACEA. ISOPODA 325 



south of Scotland where they are sometimes numerous ; 

 the principal genera are Pygocephalus, Crangopsis, An- 

 thrapakemon, and Tealliocaris. In the Upper Devonian 

 Pakeopalcemon is found, and may belong to this group. 



ORDER VI. ISOPODA 



In the Isopods the body is usually flattened dorso- 

 ventrally. There is no cephalo-thoracic 

 shield, but the first thoracic segment 

 (occasionally also the second) is fused 

 with the head. The eyes are sessile. 

 The first pair of thoracic appendages 

 are maxillipedes, the other seven are p . g ^ Arch(Eonis , 

 walking legs. The abdomen is often ens brodiei, from 

 short, and some or all of its segments Slightly reduced. 

 are fused together and with the telson. 

 Some of the abdominal appendages function as gills. 



Many Isopods are marine, but some are found in fresh 

 water, whilst a few live on land (e.g. the wood-louse, 

 Oniscus ctsellus). Many forms are parasitic, and infest 

 fish and Crustacea. 



Fossil Isopods are rare. Oxyuropoda, from the Old 

 Red Sandstone of Kiltorcan (Ireland), is probably an 

 Isopod; and an imperfect specimen, from the Old Red 

 Sandstone of Hertfordshire, described under the name of 

 Prcearcturus gig as, perhaps belongs to this group. Arthro- 

 plemxi, from the Coal Measures, has been referred to the 

 Isopoda. Undoubted examples of this Order are found in 

 Jurassic and later formations : Cy do splicer oma in the 

 Great Oolite and Purbeckian ; Archceoniscus in the Pur- 



