THE AQUARIUM. 99 



Among the bivalves, a specimen or two of the cockle-like 

 SphcBriwn corneum and half a-dozen of the pea-shells {Pisidiimi) 

 may be included. At the outside not more than a couple of 

 dozen molluscs altogether should be introduced at once, since, if 

 their surroundings are suitable, they increase very rapidly. The 

 tendency with beginners is to overstock with animal life. To pre- 

 vent disappointment, it is better to err on the other side, for excess 

 of vegetable life is less likely to give bad results than too many 

 animals. When the crop of confervae grows so fast that the snails 

 cannot keep it within bounds, we have found an old tooth-brush 

 fastened to the end of a glass rod a very useful weapon in cleaning 

 the glass. After snails, a few beetles should be taken, but only 

 the smaller lynds, since several of the larger ones are very 

 voracious. This is particularly the case with Dytiscus marginalise 

 the common large pond beetle, which both in its larval and perfect 

 state of development, should be avoided, for if introduced, it 

 would speedily devour every other living animal. Another 

 voracious beetle, smaller than D. marginalise is Z?, {Ilibias) ater^ 

 a shining black species, which should also be given a wide berth. 

 Exception in the bannings of the large beetles must be made in 

 favour of the largest, the great water beetle, Hydrophilus piceus, 

 which is entirely harmless. It is not very common, but occurs in 

 the neighbourhood of London. It may be known by its appear- 

 ance and size, resembling a female stag-beetle, and by the beauti- 

 ful play of colours shown on the elytra when swimming in the 

 water. Another harmless and very active beetle, constantly on 

 the move, somewhat resembles a flattened acorn in size and shape, 

 is Agabus agilis. A large flattened beetle^ Acilius canaliculatus, is 

 also to be recommended, for although carnivorous, it will not 

 generally attack living prey if it can find dead animal matter. It 

 therefore becomes a very useful scavenger. Most of the smaller 

 beetles may be included ; among others, Agabus maculatus, which 

 has blackish elytra with white spots, and also Laccophilus minutus^ 

 The small oval Hyphidrus ovatus, always on the move, is also an 

 attractive insect in captivity ; with it may be classed Cnemidotus 

 ccesus, which may be known by its grooved and dotted wing-cases 

 of yellow colour. A few whirligig beetles — the Httle black Gyrinus 



