vii DISTRIBUTION OF CRAYFISHES 21 3 



tliat they are part of a relict marine fauna. It would appear 

 that they have been differentiated in the lake itself during 

 a long period of isolation. 



Two groups of Brachyura, viz. the Thelphusidae and the 

 Sesarminae (a sub-family of the Grapsidae), are fresh - water. 

 Thelpliusa JluviatUis is an inhal3itant of Xorth Africa, and 

 penetrates into the temperate regions of the Mediterranean, and 

 is said to be exceedingly common in the Alban Lake near Rome. 

 Both these families have representatives on land, e.g. Fotamo- 

 enrcinus in Central and South America, and certain species of 

 Sesarma, and the closely related Gecarcinidae of the West Indies. 



The remaining families to be dealt with are the two Cray- 

 Hsh families — the Astacidae and the Parastacidae — which 

 live in rapidly moving rivers and streams, and occasionally in 

 lakes. A few species of both families have taken to a 

 subterranean mode of life, and excavate burrows in the earth, 

 e.f/. the Tasmanian Crayfish, Engaeus fossor. The distribution of 

 the Crayfishes has long engaged the attention of naturalists. 

 It was first seriously studied by Huxley,^ and has subsequently 

 been followed up, especially in IS^orth America, by i'axon "-^ 

 and Ortmann,^ but our knowledge of the South American and 

 Australian forms is still very incomplete. The Astacidae in- 

 habit the northern temperate hemisphere, the Parastacidae the 

 southern temperate hemisphere, the tropical belt being practi- 

 cally destitute of Crayfishes. Of the Astacidae the genus 

 Astacus (Potamohms), including the common Crayfishes of 

 Europe, occurs in Europe and in North America west of the 

 Rockies. The genus Camharoides, which in certain respects 

 approaches Cambarus, is found in Japan and Eastern Asia. The 

 very large genus Camhariis, on the other hand, only occurs in 

 North America east of the Rockies, so that Camharoides occupies 

 a very isolated position. The occurrence of a Camharus, C. 

 sti/r/ius, in the caves of Carniola, has been recorded by Joseph, so 

 that it would appear that this genus had a much wider range 

 formerly than now. 



Of the southern temperate Pai-astacidae, Australia and 

 Tasmania have tlie genera Astacoj'ts'is and Entjacus ; New Zealand 



' Tlie Crayfish, Internat. Scient. Series. 



- Mrni. Harvard. Mus. x., 1885. 



3 Proc. Aiucr. Phil. .Soc. xli., 1902, p. 267, and xliv., 1905, p. 91. 



