116 



MR. G. W. SMITH AXB DR. E. H. J. SCHUSTER ON 



and east. Thus P. hicarinatus i& now found everywhere on the 

 Australian continent, but it has not been able to reach New 

 Guinea or Tasmania, since these countries were cut off from the 

 mainland when P ."bicarinatus arrived. 



The relationships of the Australian Parastacidse should there- 

 fore be expressed as follows, in place of the diagram given in the 

 former memoir : — 



Cliaraps. 



Farcich(BrapSi 



Enfftetis. 



Astacopsis. 



.0^' 



Coinlnoii aucestoi'i 



2. Strilctitre^ Hahits, and Tntero'elationsMps of the 

 Species o/Enggeus. 



The several species of Engceus are characterised by the great 

 depth of the carapace, measured dorso-ventrally, and by its arched 

 roof-shaped form. The eyes are small, and the abdomen tends to 

 be reduced in size. We also note a greater hairiness of the mouth- 

 parts and neighbouring regions, a feature probably correlated 

 with the necessity of filtering the water in their burrows, which 

 is usually very muddy. The gills, especially the arthrobranchs, 

 tend to be reduced in size, and though the gill formula may be 

 the same as in the other Australian Parastacidse, yet in certain 

 forms the last pleurobranch is altogether suppressed. This entire 

 suppression of a gill, which is unknown in any of the other 

 Australian Crayfishes, is only one example of the very striking 

 morphological changes which the species of Engams may exhibit ; 

 and it may be truly said that the species of this genus are often 

 separated from one another by greater differences than those 

 which distinguish the genera of the Pal-astacid* from one another. 



