HEDRIOPHTHALMATA. 



557 



rus) and Shore-hoppers (Orchestia), being more or less amphibious 

 in habit, while some genera (such as Gammarus) possess both 

 marine and fresh-water representatives. The geological history of 

 the Amphipods is still very imperfectly known. If the incompletely 

 understood Palaeozoic genera Gampsonyx, Palceocaris, Palceorchestia, 



Fig. 417. — Gammarus locusta, enlarged about four times. Recent. (After Spence Bate 

 and Westwood). 



and the forms allied to these, be excluded from the order, the num- 

 ber of known fossil forms of the order is very small. If the genus 

 Necrogammarus described by Dr Henry Woodward from the 

 Silurian rocks of Britain, be truly referable here, it is the oldest 

 type of the Amphipods at present known. Hardly less certain is 

 the position of the Permian genus Prosoponiscus {Palceocrangori), 



Fig. 418. — Prosoponiscus {P alaocrangoti) problematicus, viewed from one side, and partially 

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



which Mr Spence Bate has described as related to the living Phcedra 

 antiqua. The fossil forms of the Amphipoda from the Tertiary 

 series are almost exclusively from fresh-water deposits, and either 

 belong to such existing genera as Gammarus or are nearly allied to 

 other living types. No fossil representatives of the little group 



