FRESHWATER CHATFISHES OF AUSTRALIA. 151 



their haVtits ari'l anatomical structure they have probably been 

 derived directly from Pamcharraps, some of the species graduating 

 very perfectly into the latter genus. The burrowing land-Cray- 

 fishes are therefore the most modern product of all. Although 

 it is not mv intention to give any description of the genus Engceus 

 iu this paper, cei-tain of their characters may be mentioned 

 which point to their high degi'ee of sjie^ialization. Their whole 

 appearance ami form are very diflFerent from the other Crayfishes, 

 the thorax being extremely deep dorso-venti-ally, a character 

 which is correlated with the very great size of the gastric mill. 

 The abdomen is much reduced in size. Cei-tain of the species 

 show a peculiar re<:luction in the gill-formula, the last pleuro- 

 branch V)eing entirely absent, while in some species the exoj:>odit^ 

 of the maxillipede is completely absent. The body is entirely 

 free from spines or ridges. 



The above survey and an examination of the diagnoses of the 

 several genera afford a remarkably clear picture of the evohi- 

 tionarv relationships of the genera described, which we may 

 repi-esent in the form of a table, thus : — 



Charnjis. Enpems. I'arachcfrapt. 



Common ancestor. 



AVe mav attempt to reconstruct also the means by which these 

 genei-a have reached their present geographical distribution. The 

 outstanding fact is clearly the complete isolation iu structure and 

 distribution of the western Charaps from the eastern Astacopsis. 

 This sharp distinction between a South-eastei-n and a North- 

 western Australian fauna does not apply only to Crayfishes, but 

 is practically uni^•ersal. and in looking for a cause the attention 

 of naturalists has been called to the existence in Central Australia 

 of Cretaceous deposits, which are held to indicate the existence of 

 a sea in Cretaceous times dividing Australia into a western and 

 an eastern island. ^Ve may therefore start with the hypothesis 

 that the separation and divergent evolution of Astacopsis and 

 Chcpraps dates from the separation of Eastern and AVestem 

 Australia V.v the Cretaceous sea. It would appear that, with the 



