Rostrum (= labium + labial palpi) rather long but very weak and pale, consisting of two or three segments inclusive of the unpaired basal segment. Genal edge of head always produced downwards into a triangular process situated behind the insertion of the maxillæ at the ventral oral angle. Thoracical tergites together shorter than first abdominal tergite. To this group belong the chigoes and their allies, the most truly parasitic fleas. About fourteen species have been described, which can be grouped into three genera, viz. _Dermatophilus, Hectopsylla, Echidnophaga_. II. Family _Pulicidæ_. Taschenberg (1880). Rostrum (= labium + labial palpi) more or less strongly chitinized, consisting, except in a few cases, of five, or more, segments inclusive of the unpaired basal one. Thoracical tergites together longer than first abdominal tergite. Here belong the majority of _Siphonaptera_. III. Family _Ceratopsyllidæ_. Baker (1905). Head on each side with two flaps situated at the front oral corner. Here belong the bat-fleas only. There are several genera, and about twenty-five species have been described. In most of the bat-fleas the maxillæ are shaped like a dumb-bell, but in the genus _Thaumopsylla_ they are triangular as in the _Pulicidæ_. * * * * * Oudemans (1909) has put forward an alternative classification of the order _Siphonaptera_ based on the morphology of the head:—I. _Integricipita_, II. _Fracticipita_. APPENDIX B A LIST OF BRITISH FLEAS AND THEIR HOSTS A list of the British Fleas (_Siphonaptera_) revised to March 1913: Names Usual Hosts PULEX, _L._ P. irritans Man, Badger XENOPSYLLA, _Glink_. X. cheopis, _Rothsch._ Rat ARCHÆOPSYLLA, _Dampf_ A. erinacei, _Bouché_ Hedgehog CTENOCEPHALUS, _Kolen_ C. canis, _Curt._ Dog C. felis, _Bouché_ Cat SPILOPSYLLUS, _Baker_ S. cuniculi, _Dale_ Rabbit ORNITHOPSYLLA, _Rothsch._ O. lætitiæ, _Rothsch._ Puffin and Manx shearwater (Scilly Is. only)