A Synopsis of the Recent British Ostracoda. 113 



of the last-named author, embodied in his recent work on the 

 marine Ostracoda of Norway,* are of particular interest to 

 English naturalists, seeing that the marine faunas of the two 

 countries exhibit a very close affinity. He has succeeded, after 

 a most careful and painstaking investigation of the Norwegian 

 species, in accurately ascertaining the minute structure of 

 animals belonging to all the described fossil genera (excepting 

 only, as he says, Cytheridea, which, however, I regard as 

 belonging to the same genus as the forms he describes under 

 the name Cyprideis) , and has also established a large number of 

 new species and genera. After a similar survey of the British 

 marine and fresh-water species (in which I have received 

 most valuable assistance from many collectors and naturalists 

 whom I need not here stop to name), I have myself added many 

 species to the list, and have found it needful also to propose 

 some few new genera. A brief analysis of these I propose 

 now to lay before the reader ; but before doing so, it will be 

 desirable to describe succinctly the general type of structure of 

 the Ostracoda. 



Each member of the class Crustacea is considered, typically, 

 to be divisible into twenty-one annular segments, or somites, 

 seven of which belong to the head (cephalon), seven to the 

 thorax (pereion), and seven to the abdomen (pleon). But it is 

 only in very few cases, and these amongst the more highly 

 organized members of the class, that these segments, or their 

 rudiments, are discernible. In most cases some of the seg- 

 ments are fused together, so that their real nature is to be 

 recognized only by the presence of certain limbs or appendages 

 which indicate their existence. Thus, under the hard, cal- 

 careous carapace, or shield, which protects the head and back 

 of the lobster, we find gathered all the cephalic and thoracic 

 members of the animal, and so we learn that the great dorsal 

 buckler consists, in fact, of all the cephalic and thoracic 

 segments of the body, cemented into one strong protecting plate. 

 This principle, infinitely modified, runs through the whole class ; 

 but we also find that, in addition to the coalescence of various 

 segments, other segments are often entirely absent, their 

 presence not even indicated by the existence of any limbs or 

 appendages. So that it is only by the careful study and 

 comparison of the whole group that the real nature and 

 homologies of any particular organ can be made out. The 

 appendages of the twenty-one segments of the typical Crus- 

 tacea may be tabulated as follows : — 1, Eyes; 2, 3, First and 

 second antennas ; 4, Mandibles ; 5, 6, Two pairs of jaws ; 

 7, 8, 9, Foot-jaws; 10 — 14, Ambulatory legs ; 15 — 20, False 

 or abdominal feet ; 21, Tail-piece or post-abdomen. In the 



* " Oyersigt af Norges Marine Ostracoder " af G. O. Sara, 1865. 

 VOL. XII. — NO. II. I 



