Curr. ros.—Ón Astacus fluviatilis and Paranephrops setosus. 153 
f the earth's history, a Crustacean, similar to Paranephrops 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 Palemon or Peneus 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 Homaride are represented in the Indian Ocean only by 
Nephropsis.’’* : 
The comparison of the male reproductive organs in Palinurus, 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 Paranephrops is thus in all probability more nearly akin to 
Palinurus than it is to Astacus, yet it is curious to notice that in general 
appearance it resembles Astacus much more nearly than it does Palinurus. 
Iam not referring to the absence of chelate limbs in Palinurus, for they 
must obviously have been lost after the Parastacide branched off, but to the 
size, the colour, the shape of the antennules and antenne and their size 
relatively to that of the animal, the narrow thoracic sterna, the movability 
of the last thoracic 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 can be due to the same cause. 
My observations with regard to the gastroliths or ''crab's eyes”’ of 
Paranephrops 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 gastrie 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. 
