I1gO CRUSTACEA—-EUCARIDA—DECAPODA CHAP. 
the same thing, though not to the same extent, was happening 
in female Crabs. Weldon supposed that this change might be 
correlated with the silting up of Plymouth Sound and _ the 
consequent fouling of the water. To test this hypothesis he 
kept a very large number of male Crabs in water to which fine 
porcelain clay was added and kept in continual motion. In the 
course of the experiments the survivors and the dead were 
measured, and it was found that the mean carapace-breadth of 
the survivors was less than that of those that succumbed. The 
experiment was repeated with the fine sand that is deposited 
and left at low water upon the stones on Plymouth beach, and 
the same result was observed. It was also noticed that the 
individuals which died had their gills clogged with the sand, 
while those that survived had not. As a further confirmation, 
a great many young male Crabs were isolated and kept in pure 
filtered water, and they were measured before and after moulting ; 
these measurements, when compared with measurements of the 
frontal breadth in Crabs of the same size taken at random upon 
the beach, were found to show a greater breadth than the wild 
Crabs, thus indicating that a selection of narrow Crabs was 
taking place in Nature which did not take place when the 
Crabs were protected from the effects of fine sand in the 
water. 
The whole chain of evidence goes to show that the carapace 
breadth in Carcinus maenas in Plymouth Sound is being influ- 
enced by the rapid change of conditions occurring in the locality. 
Various objections have been urged against this conclusion, but, 
though they merit further investigation, they do not appear very 
weighty. 
The fresh-water Crab, Thelphusa fluviatilis, common in the 
South of Europe and on the North coast of Africa, belongs to 
the Cyclometopa, and is interesting from its direct mode of 
development without metamorphosis. 
Fam. 1. Corystidae.—The orbits are formed, but, unlike all 
the other families of the Cyclometopa, are incomplete. The 
body is elongate and oval, and the rostrum and front edge of the 
mouth rather as in the Oxyrhyncha, in which Tribe they are 
sometimes included. Corystes, with a few species in European 
seas. C. cassivelaunus at Plymouth. 
Fam. 2. Atelecyclidae.—Perhaps related to the foregoing. 
