if the first which constitutes the head be allowed to count as a single segment notwithstanding its evidently composite character. The follow- ing five segments are thoracic, and the terminal five form the pleon, a part so called not with regard to the Copepoda but to other crustaceans in which it is in fact what the word implies, the swimming factor of the organism. In the Copepoda this generally ends in a pair of seti- ferous processes known as the caudal fork or furca, but otherwise it has no appendages, and the expression Gymnoplea means ' those that have a naked pleon,' or in other words a pleon without appendages. Contrasted with these are the Podoplea, or ' those that have feet on the pleon.' But the awkward thing is that these do not really any more than the others have such feet. What they do have is this. The last segment of the thorax, instead of keeping with its own company, has in a manner broken away, and tacked itself on to the group of pleon seg- ments so that it gives the Podoplea not the reality but the look of having a pair of feet (often very rudimentary ones) on the pleon. To this division belong the family Cyclopidas, of which Cyclops fuscus (Jurine) and C. bicuspidatus, Claus, were obtained respectively from Burnham Beeches and a common adjacent thereto, and the family Ar- pacticidae, of which Canthocampus pygmceus, Sars, was obtained from both the last mentioned localities, and C. stapbylinus (Jurine) from Burnham Beeches and Stoke Park. In the Diaptomidas one antenna, either the right or the left, of the first pair is modified into an organ for clasping the female, a geniculation being formed between the eighteenth and nineteenth joints. In the Podoplea either both antennas of the first pair geniculate or neither does. The genera Cyclops and Canthocampus agree in the character of having a pair of clasping antennas. The marks of separation between the two families to which these genera respectively belong are rather too complicated to be conveniently dis- cussed in this chapter. It may however be noted that, while the two species of Cyclops above named both have the first antennas seventeen- jointed, none of the Arpacticidae have more than ten joints in these appendages ; in Canthocampus staphylinus they are eight-jointed.
When allowance is made for the scarcity or absence of any pub- lished information on the subject, the crustacean fauna of this county may now claim not only to have shown excellent promise for future researches, but to have given an earnest of success by already accom- plished discoveries of unusual interest. In adding three species at once to the rather limited number of English terrestrial isopods Bucking- hamshire will not easily be rivalled by other counties.
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