lxxxiv 



THE VOYAGE OF H.M.S. CHALLENGER. 



streams and lakes of the continents and larger islands of both the northern and southern 

 hemispheres. 



The northern genera differ in the number of their branchiae from those of the 

 southern, which also exhibit evidence of a divergence in descent. 



Those of Europe and America possess evidence of a closer consanguinity, but are 

 generically separated by the numerical distinction of the branchial plumes; whilst those 

 of Eastern Europe and Western Asia differ only in specific value very considerably from 

 those of Western Europe, those of Eastern Europe differ in external form but slightly 

 from those of Eastern America. 



Those that inhabit the rivers of South America differ in external appearance from 

 those of the northern continent, but bear a corresponding resemblance to those that 

 inhabit the rivers of Australia, while these latter differ widely both in appearance and 

 structure from the New Zealand forms. The solitary species of the only genus of 

 Madagascar differs structurally from, but corresponds externally with, those that belong- 

 to Australia. 



In all the northern forms the outer margin of the dactylos is either straight or 

 incurved, whereas in the southern genera the same part is always arched or curved 

 outwards. It is but a small distinction, but it is one that is invariably capable of deter- 

 mining the northern from the southern species, and it is interesting to notice that a fossil 

 species was recently found in the Eocene formation of North America that possesses this 

 congenital feature peculiar to the northern forms ; a circumstance that demonstrates the 

 long and persistent character in the history of this feature. 



The several species have been arranged in accordance with their general form and the 

 number of their branchiae into the following genera : — 



Astacus in Europe and Asia. 

 Astacoides in Madagascar. 

 Astacopsis in Australia. 

 Cambarus in North and East America. 



Cherops in Tasmania. 

 Engeeus in Tasmania. 

 Paranephrops in New Zealand. 

 Parasiticus in Australia and South America. 



In looking back through past ages it would appear that the earliest macrurous forms 

 are those that have been obtained from the coal measures of Shropshire and Glasgow, and 

 if the illustrations given be anatomically correct, the structure of Palseocarabus corre- 

 sponds with a Crustacean that belongs to the Trichobranchiata. The short and stunted 

 form of the scaphocerite of the second pair of antennae is such as is seen to exist in 

 the Astacidea of recent periods, but the restoration of the specimen of Palseocarabus 

 russellianus, as given in Salter's paper, 1 shows that the rostrum is long and laterally 

 compressed, contrary to its character in the Astacidea, or in fact in any of the 

 Trichobranchiata, excepting those of the family Stenopidse. 



1 Quart. Journ. Geol. Soc, vol. xix. p. 520, figs. 1, 2. 



