150 



ilR. GEOFFREY SMITU OX THE 



f:ict tliat the last posterior artbrobraucli is not rudimei)tary. Its 

 membei-s inhabit running water, and they are confined in dis- 

 tribution to the west and north coasts and to Xew Guinea, being 

 eutii-ely absent from the southern and eastern districts occupied bv 

 Astacopsis. In the south the aiid coast-line, fringing the Great 

 Australian Bight, constitutes a wide and insurpassable barrier 

 between the genera Astacopsis and Chferaps. On the ea^t coast 

 Chferaps is absent and Astacopsis does not appear to occur much 

 north of Svdnev. 



The species C. interrnedius (PL XXIV. fig. 2) from Western 

 Australia is of great interest, because it forms a perfect 

 transition to the genus Parachteraps. C. intermedins retains the 

 diagnostic features of a Chceraps. but it presents a remarkable 

 approach in general facies and in a number of points to Para- 

 chtrraps hicarinatxs. 



The genus PARACH^RArs. consisting of the single species P. blcar- 

 inafus (PI. XXI.), is closely allied in all its features to Chcpraps, 

 and the alliance is made . more obvious by the existence of 

 the before-mentioned C. intermedius, which may reasonably be 

 regarded as the ancestral form from which P. hicarinatus has 

 been derived. P. hicarinatus is the most widely distributed 

 Ci-ayfish on the Australian continent, being the only form which 

 penetrates into the interior of the continent. It is, in fact, 

 universjilly distributed all over the continent, occuiTing with 

 Chceraps in the west and north and with Astacopsis in the south 

 and east. It does not occiu' in Tasmania or in New Guinea. The 

 relationship of this genus and species is obviously with the 

 western Ckceraps. and it is also equally certain that it is a secon- 

 daiy derivation from Chceraps, which has been able to spread 

 across the deserts eastwards and invade Victoria in the south 

 by becoming adapted to live under semi-desert conditions. The 

 alternative supposition, viz. that Parachceraps is the ancestral 

 form and has given rise to Chceraps, is clearly negatived by the 

 impossibility of thus a<?counting for the fact that, although it 

 ranges as a continuous species all over the continent, it should have 

 given rise to several species of Ckceraps only in the west. It would 

 be difficult also on this supposition to account for the fact that 

 Ckceraps difiers less from Astacopsis than Parctckceraps does. In 

 fact, if we take the now almost universally distributed Para- 

 ckceraps as representing the ancestral form of Australian Cray- 

 fish, the entire isolation, both structui-ally and geographically, of 

 Astacopsis and Ckceraps becomes unintelligible. 



It is worthy of notice, also, that Parack<eraps apparently does 

 not occur either in Xew Guinea or Tasmania, and. although stress 

 cannot be laid on this fact, it is possibly another indication of 

 the modem origin of Parackceraps, after the separation of Xew 

 (Tuinea and Tasmania from the mainland of Australia. 



The genus Exg^us, comprising the land-burrowing Crayfishes, 

 which have gone a step beyond Parackceraps in their independence 

 of water, are confined to Victoria, Gippsland. and Tasmania. From 



