326 CRUSTACEA. AMPHIPODA 



beckian (fig. 142) ; Palcega in the Cambridge Greensand, 

 the Lower Chalk, and foreign Tertiary ; and Eosphceroma 

 in the Oligocene of the Isle of Wight. 



ORDER VII. AMPHIPODA 



The Amphipods {e.g. Gammarus, Talitrus) are usually 

 of small size, and generally the body is compressed from 

 side to side. Just as in the Isopods, there is no cephalo- 

 thoracic shield, and the first thoracic segment (sometimes 

 also the second) fuses with the head. The first pair of 

 thoracic appendages are maxillipedes ; the appendages of 

 the seven free segments bear the gills, and are divisible 

 into an anterior group of four in which the terminal parts 

 of the legs are directed backwards, and a posterior group 

 of three in which the terminal parts are directed forward. 

 The abdomen is usually elongated and carries six pairs of 

 appendages ; the three anterior serve for swimming, the 

 three posterior for jumping. The eyes are sessile. 



Some of the Amphipods are marine, others live in 

 fresh water. The marine forms have a wide distribution, 

 and are very numerous, especially in shallow water, and in 

 Arctic and Antarctic seas. 



Fossil Amphipods are very rare. Necrogammarus from 

 the Lower Ludlow rocks of Leintwardine has been referred 

 to this group, but its systematic position is uncertain — 

 Peach thinks that it may belong to the Myriapoda. 

 Other forms, whose affinities are likewise uncertain, have 

 been recorded from the Carboniferous and Permian ■ de- 

 posits. Undoubted Amphipods are found in the Tertiary 

 formations and belong mainly to genera which are still 

 existing (e.g. Gammarus from the Miocene). 



