120 THE VOYAGE OF H.M.S. CHALLENGER. 



of a specimen or figure excepting those that have been restored in which it has been 



determined. 



Taken as a whole, the specimen that I have here described resembles the form of the 

 recent Polyclieles as nearly as it does that of the type of the ancient Eryon. But in the 

 breadth of the pleon and the absence of the dorsal carina, it exhibits a condition that 

 demonstrates it to be no very distant departure from the genus Astacus, to which the 

 great chela, notably in Cambarus simulans, Faxon, and Cambarus clarldi, Bajer, bears a 

 near and characteristic resemblance, and the likeness would be more apparent if the 

 animal, instead of being dorsally depressed, had, like Astacus, a more rounded or laterally 

 compressed form. 



It appears to me that the family of the Eryonidse was a departure under deteriorat- 

 ing circumstances from some marine ancestor of Astacus, and that the recent genera are 

 in direct descent from the Archeeastacus of the European Lias. 



The fossil genus Palzeocarabus, from the Glasgow and Shropshire coal measures, 

 appears to possess characteristic forms in the several genera of Arctus, Polyclieles, 

 and to be represented most closely by the recent genus Synaxes, from which it seems 

 to differ chiefly in the laterally compressed rostrum, and it is interesting to notice that, 

 separated as these genera are in time, as widely as the pex-iod when the coal-plants 

 were living and growing in their native soil is from that of the present day, there is 

 very little beyond specific distinction in character separating the oldest fossil from 

 the most recent Macrurous Crustacea of the same family, and if we are, as is but 

 reasonable, to judge of the alteration of parts unknown from the parts that are known, 

 there is very little variation in structure also. So that in this group of animals whatever 

 specific changes may have successively been produced, they are small in degree and 

 unimportant in character ; and therefore we may assume that the conditions of life on 

 the globe, so far as relates to the present class of animals, can have undergone but little 

 change. 



Geographical Distribution. — The recent genera that belong to this family are widely 

 distributed, but all of them appear to require certain conditions of depth, temperature, 

 and character of sea bottom. 



The genus Polyclieles has been taken in the Mediterranean, and in the Atlantic off 

 the coast of Spain f in the West Indies, and in the longitude of the Fiji and Kermadec 



1 In the Quart. Journ. Geol. Soc, vol. xxii. pi. xxv. fig. 1, Dr. Woodward delineated " by the help of the fine 

 examples in the cabinet of the Rev. P. B. Brodie, F.G.S., and those in the British Museum," a completely restored 

 figure of Eryon barrovensis (M'Coy) in which the scaphocerite is fixed at the extremity of a peduncle that is independent 

 of that of the antennas. This condition not being in accordance with the anatomical structure of the Macrurous 

 Decapoda, I am induced to think that the small pedicular plate at the extremity of the third pair of maxilke is in- 

 tended, of which a drawing is given at fig. 31, p. 135, in this Report, and which in some recent species extends beyond 

 the frontal margin. It may be seen on PI. XIX. fig. C", which represents the under surface of the head in JVillemmsia 

 leptodactyla. 



2 Norman, On the Willemoesia group of Crustacea, Ann. and Mag. Nat. Hist., ser. 5, vol. ii. p. 384, 1878. 



