CRUSTACEANS circumstance the sea-shrimp.' ' In this genus the first trunk-legs are so far from being developed into monstrous claws that they were long de- scribed as simple, a technical expression for limbs that have no grasping power. But Dr. Caiman has recently pointed out that they do in fact end in a thumb and finger, though the chela so formed is of only micro- scopical proportions. Very commonly parasitic on this prawn is another crustacean, an isopod called Hemiarthrus abdominalis (Kroyer), which may be discussed at another opportunity. Its occurrence might have been taken for granted, but it is actually reported by Metzger.* Not all prawns find salt water a necessity of existence. There are many which are able to live at will also in brackish water or fresh. Among these is Palamonetes varians (Leach), which may be met with in several English counties. The genus is separated from Leander by the absence from its mandibles of the so-called palp. The very same distinc- tion between nearly allied genera is met with in other parts of the class Crustacea, and in some of the instances it is difficult to understand why one set of forms should discard what the other set retains. In Palamo- netes varians^ it should be further observed, the frontal sword or rostrum is straight, entire at the tip, with two teeth on the under margin, and four to six on the upper. Its occurrence in Norfolk is noticed by Bell ; ' but there is, I imagine, an indirect reference to it also in the following argu- mentative and singular record, to which my attention was called some time ago by Mr. Whitaker, lately president of the Geological Society. The passage is in Mr. S. B. J. Skertchly's Memoir on Fenland.* It is headed ' Living Prawns in the Silt.' 'In the summer of 1873 I investigated a curious case of the en- tombment of the ova of prawns in the marine silt for a lengthy period. Mr. S. H. Miller directed my attention to the case, and accompanied me on my visit. The facts were communicated to Land and Watery and specimens were got which lived for a long time in the Brighton Aquarium. ' At Walsoken brickyard, near Wisbech, pits are sunk in the clay, and in the year 1859 a bed of fine sand or silt was pierced at a depth of fifteen feet. From this bed a strong salt spring rose, the water of which was much more saline than that of the river in the neighbourhood, and this, mixing with the fresh water in other parts of the pit, rendered it so brackish as to kill the pike, though the carp, tench and insects seemed unaffected by the change. Shortly after this incursion of salt water prawns began to appear in the pit, and the supposition is that their ova were embedded in the marine silt and kept alive by the salt water with which the bed was charged, and that the recurrence of favourable conditions of open water and light enabled them to hatch, and since that time they have abounded in the pits. The largest individuals are about ' Malacoitraca Podophthalmata Britannia, text to pi. 40, March, 1815.
- Nordseefahrt der Pommerania, p. 286.
- British Stalk-eyed Crustacea, p. 310.
- pp. 241-243, 1877, reprinted in Geol. Survey Mem., sheet 65, p. 135, 1893.
189
