Messrs. Jordan and Rothschild in a “Revision of the Sarcopsyllidae” (University of Liverpool 1906) criticise Baker’s classification, and reduce the families to four, extending the limits of the Family SARCOPSYLLIDAE, and adding seven new species. The flea differs from most other insects in having the whole wedge-shaped body vertically flattened. It is admirably adapted for crawling through hair or feathers, and the large stout spiny legs are well suited for jumping. The head, indistinctly separated from the body, is short, furnished with jointed antennae situated above but behind the eyes; the mouth is produced into a stout pointed proboscis with which it punctures the skin and sucks up the blood of its host. They are all of a more or less reddish brown tint, clothed with scattered stout bristles, and the abdomen is rounded at the apex; the legs are furnished with a pair of tarsal claws. The fact that fleas are capable of spreading the germs of plague and even leprosy has caused a great deal of attention to be devoted to this group, and they have during the last few years been sought for and collected from all parts of the world. Two species are common in the house in Australia, of which the “domestic flea,” _Pulex irritans_, is too well known to need much description. They deposit their eggs, which are tiny ribbed crystalline spheres (very beautiful objects under the microscope) in the dry dust in cracks and crannies in the floor, or in the corners of badly-swept rooms. From these eggs hatch out slender, legless, transparent grubs with several short bristles on the anal extremity; these grubs feed upon the dust and, when full grown, spin a silken tube in which they pupate buried in the dust. _P. serraticeps_ is known as the dog and cat flea, though it is not uncommon at times in the house where animals are running about; but though it sometimes comes on man, it is an accidental infestation, and it gets away to its natural host as soon as it can escape. It can be easily distinguished from the common house flea by its more elongate form, and by the black comb-like spines fringing the back of the head and the first thoracic segment, which are absent in the former. The Rat and Mouse Flea, _P. fasciatus_, is a paler coloured, more slender flea, also with a very extended range over the world. It is notorious as the species that, when living upon plague-infested rats, can transmit bubonic plague to man. Denny (Annals and Mag. Nat. Hist. XII. 1843) has described another species, which he placed in this Genus, obtained from Tasmania and found upon the _Echidna_, and which he has called _Pulex echidnae_. About a dozen indigenous species have been recorded from Australia.