388 ARTHROPODA. 



limbs. Abdomen well developed, and composed of seven segments. 

 Seven pairs of thoracic limbs, directed partly backwards and 

 partly forwards. 



The most familiar recent forms of the Amphipoda are the 

 " fresh-water Shrimps " (G-ammarus), the Sand-hoppers (Tali- 

 trus), and the Shore-hoppers (Orchestia). The oldest repre- 

 sentative of the order is a doubtful form, which has been 

 described by Mr Woodward from the Upper Silurian rocks 

 under the name of Necrogammarus. The Carboniferous genus 

 Gampsonyx has been referred here, but is perhaps more pro- 

 perly placed amongst the Stomapoda, or among the Isopoda. 

 From the Permian rocks, however, Mr Spence Bate has 

 described an Amphipod to which he has given the name of 

 Prosoponiscus prollematicus (fig. 243), and which he believes 

 to be very closely related to the living Phcedra antiqua. 



Fig. 243. Prosoponiscus problematicus, viewed from one side, and partially restored. From 

 the Magnesian Limestone (Permian) of Durham. (After Spence Bate.) 



OEDER ISOPODA. 



Crustaceans in which the head is distinct from the segment 

 bearing the first pair of feet. The eyes are compound and 

 sessile. There are usually 'seven pairs of thoracic appendages, 

 borne upon seven movable segments. The animal sometimes has 

 the power of rolling into a ball. The abdominal segments are 

 coalescent, and form a broad caudal shield, beneath which the 

 branchial are carried. 



Of the living Isopods, some (Bopyridm) are parasitic in 

 their adult condition upon other Crustaceans. Others, such 

 as the common Wood-lice (Oniscus), live habitually upon the 



