148 MU. GEOFFREY SMITH ON THE 



hU cases it bears a certain number of well-developed branchial 

 filaments. 



" The first abdominal somite possesses no appendage in either 

 sex ; and the appendages of the four following somites are large. 

 The telson is never completely divided by a transverse suture. 



" More or fewer of the branchial filaments are terminated by 



short hooked spines ; and the coxopoditic setae, as well as those 



which beset the stems of the podobranchise, have hooked apices." 



The Astacidse possess the converse of these diagnostic 



cliaracters. 



The Astacidie and Parastacidse, the one familj^ occurring in 

 the Northern Hemisphere, the other in the Southern, are there- 

 fore sepn.rated by important characters, and it is very probable 

 that they have been independently evolved fi'om marine lobster- 

 like ancestors which already differed in these charactei-s before 

 they took to a fi-esh\\ater life. 



The occurrence of Parastacidae in Australia. New Zealand, and 

 South America, with an aberrant genus [Astacoides) in Mada- 

 gascar — tha,t is to say, in countries which are now separated 

 by wide stretches of ocean — is a striking fact in geographical 

 distribution, but it does not stand alone, the distribution of 

 many freshwater fish, Crustacea., molluscs, etc., having a similar 

 character in the Southern Hemisphere. 



These facts, taken in conjunction with geological evidence, have 

 led many naturalists to assume a much greater extension of the 

 Antarctic Continent in past times which is supposed to have been 

 connected with South America, Australia, and New Zealand, and 

 possibly at a very remote period with Madagascar, thus permitting 

 the migration of la,nd and freshwater animals to and from those 

 countries. In the case of the Parastacidse the only alternative 

 theory is that the South-American, Australian, and New Zealand 

 genera have been independentl}^ derived from some common 

 marine ancestor. 



Our concern here, however, is not so much with the origin of 

 the Parastacidge in the remote past, but with the inter-relation- 

 ships of the Australasian genera and their probable evolution and 

 migrations. 



We can distinguish four genera of Australasian Crayfishes — 

 Astacopsis, Parachceraps^ Chmraps, and Engceus (the last-named 

 genus we will consider as a single entity, though it may be found 

 convenient hereafter to split it up into several subgenera). 



The members of the genus Astacopsis are characterised by the 

 development of spines or tubercles upon the body and limbs, and 

 by certain features in the gills and appendages which are fully 

 set forth in the diagnosis of the genus on p. 154. They inhabit 

 swift-moving streams and rivers ; they are not found in ponds and 

 water-holes, and they are not known to forsake the water for any 

 period of time. Their distribution is as follows: — In Tasmania 

 there occurs A. franklmii (Pis. XIV., XY.), the largest Crayfish 

 in the world. It is confined to the rivers a.nd streams upon the 



