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 A HISTORY OF ESSEX (Jurine), and M. hirsuticornis, Norman and Brady, of which Mr. Scour- field writes : ' Without doubt this is one of the best records in the whole list of Epping Forest Entomostraca. Since the species was first described in 1867, it appears to have been only once again recorded in the British Isles, 1 and on the continent it has only been found by a few observers. It is a fine species, being in fact the largest of the genus. Numerous specimens were taken in Connaught Water on March 10, 1894, but curiously enough none could be found about a month later, nor has the species been seen again.' The fourth family of this section has been called by several writers Lynceidse, a name which properly belongs to the phyllopod family Limnetidae, and which cannot in any case be used for a group in which there is now no such genus as Lynceus. The present family must be called Chydoridas, from its earliest genus Chydorus, instituted in 1 8 1 6 by W. E. Leach, who may be regarded as the founder of English carcinology. To Essex are assigned Chydorus sphcericus (O. F. Miiller),one of the excessively common species ; C. ccelatus, Schodler ; C. latus, Sars ; C. globosus, Baird. From the numerous other genera Mr. Scourfield records Eurycercus lamellatus (O. F. Miiller) ; Acroperus harpa, Baird, with a large variety perhaps equivalent to Koch's Lynceus leucocephalus ; Camptocercus rectirostris, Schodler, which is perhaps Baird's C. macrourus ; Leydigia quadrangularis (Leydig), with a name of rather doubtful validity ; Graptoleberis testudinaria (Fischer) ; Alona guttata, Sars ; A. tenuicaudis, Sars ; A. quadrangular is (O. F. Miiller) ; A. ajpnis, Leydig ; A. costata, Sars ; A. rectangula, Sars, to take the place of A. intermedia, Sars, an earlier record now cancelled ; A. rustica, T. Scott, ' only obtained by washing pieces of wet moss'; Alonella excisa (Fischer) ; A. nana (Baird), smallest of known arthropods ; A. rostrata (Koch) ; Pleuroxus trigonellus (O. F. Miiller) ; P. uncinatus, Baird ; Peracantha truncata (O. F. Miiller) ; and Monospilus tenuirostris (Fischer). Of the last Mr. Scourfield says : 'The presence of this very peculiar species in the forest district was first ascertained by its being found in the stomach of a roach from the Eagle Pond. It has since been taken with the net from the same piece of water.' The genus Monospilus, Sars, has more than one character that may well claim to arrest the student's attention. Thus M. Jules Richard observes, ' Exuviation is the rule among Cladocera, but in some cases the old carapace is not shed ; it remains on the new one, so that we have the appearance of lines of growth fringed with setae as in Ilyocryptus and Monospilus? 2 This, it will be remembered, is a regular and prominent feature in the bivalved phyllopods. Another character is enjoyed by Monospilus uniquely among the Cladocera, and to this it is indebted for its rather singular name, meaning ' with only a spot.' The rest of the Cladocera have each a single median eye, composed of more or less numerous elements, and in addition to this they sometimes have, and sometimes have not, on the ventral face of the head a small mass of 1 Scott and Duthie, Fishery Board for Scotland, th Ann. Rep. p. 229 (1896). 214
 * Annales des Science Naturelles, ser. 7, vol. xviii. p. 309 (1895).