Chilton. — On Astacus fluviatilis and Parauephrops setosus. 153 



of the earth's history, a Crustacean, similar to Parauephrops or Astacopsis in 

 its general characters, but with the first pair of abdominal appendages fully 

 formed, which we may call provisionally- Protastacus, inhabited the ocean, 

 and that it had as wide a distribution as Palamon or Penmis at the present 

 day. Let us suppose, further, that the northern form of the genus tended 

 towards the assumption of the Potamobiine, and the southern towards that 

 of the Parastacine type. Under these circumstances it is easy to under- 

 stand how such rivers as were, or became, accessible in both hemispheres, 

 and were not already too strongly tenanted by formidable competitors, 

 might be peopled respectively by Potamobiine or Parastacine forms, which, 

 acquiring their special characters in each great river-basin, would bring 

 about the distribution we now witness. As time went on, the Protastacus 

 stock might become extinct, or might be represented only by rare deep- 

 water forms, as the Homarida are represented in the Indian Ocean only by 

 Nephropsis.' 1 ''* 



The comparison of the male reproductive organs in Pdlinurus, Para- 

 nephrops, Homarus, and Astacus, appears to lend every support to this hypo- 

 thesis, and I have only to add that the Protastacus stock appears to have left 

 Palinurus, which has lost the chelate limbs possessed by its ancestors, as its 

 marine representative in the Southern Hemisphere. 



Though Parauephrops is thus in all probability more nearly akin to 

 Palinurus than it is to Astacus, yet it is curious to notice tbat in general 

 appearance it resembles Astacus much more nearly than it does Palinurus. 

 I am not referring to the absence of chelate limbs in Palinurus, for they 

 must obviously have been lost after the Parastacidce branched off, but to the 

 size, the colour, the shape of the antennules and antennae and their size 

 relatively to that of the animal, the narrow thoracic sterna, the movability 

 of the last tboracic segment and the shape of the abdominal appendages. 

 Some of these points, such as the size and colour, are no doubt due to mere 

 adaptation to surrounding circumstances, but it seems difficult to believe 

 that the other resemblances to Astacus-csui be due to the same cause. 



My observations with regard to the gastroliths or " crab's eyes " of 

 Parauephrops do not agree with those given by Professor Huxley for 

 Astacus in " The Crayfish," and I therefore mention it here referring to the 

 body of the paper for the details. He states that the Gastroliths "are 

 found fully developed only in the latter part of the summer season, just 

 before ecdysis sets in,'' and that they " are cast off with the gastric arma- 

 ture in general." I have, however, obtained specimens in September and 

 October (i.e. in the Spring) with gastroliths present. Some caught in 

 September had very small gastroliths, but one caught in October had them 



* Proc. Zool. Soc, 1878, pt. iv., p. 787. 



