147 



or quite wanting. Antennae never adapted for swimming, the anterior ones 

 generally sub-pediform and geniculate, like the posterior, the geniculation 

 occurring between the 2 segments of the basal part, the distal of which is always 

 powerfully developed, terminal part having the number of joints more or less 

 reduced and clothed with scattered setae often mingled with strong spines. 

 Posterior antennae provided at the end of the basal part with a long rod-like 

 flagellum, curving downwards in front of the terminal part, and containing the 

 efferent duct of a gland lying on each side of the front part of the body, 

 terminal part generally 3-articulate, 1st joint short, and without any sensory 

 appendages behind or any fascicle of setae at the extremity, penultimate joint 

 never, as in the Cypridae, produced at the end in front to a claw-bearing process. 

 Mandibles and maxillae on the whole built on the same type as in the Cypridae, 

 though in some few cases (Paradoxostoma) conspicuously transformed. 3 pairs 

 of ambulatory legs present, all of a rather similar structure and freely projecting 

 from the shell, the anterior pair answering to the maxilJipeds iu the Cypridae, 

 but never partaking in the mastication, nor provided with any true vibratory 

 plate. Caudal rami rudimentary and quite immobile. Germinal part of genital 

 organs not lodged within the valves. Ejaculatory tubes absent. Copulatory 

 appendages more or less complicated. Brush-like ventral appendages always 

 present in male. 



Remarks. This family comprises a vast number of various forms, both 

 fossil and recent, all of which originally were referred to a single genus, Cythere. 

 The far greater bulk of them, are strictly marine, only a comparatively small 

 number of forms being found in fresh water, and some few species exclusively in 

 brachish water. They all are of comparatively small size, seldom exceeding a 

 length of 1 millimeter and, as a rule, very much smaller. Owing to this circum- 

 stance, and to the specimens being in most cases picked up from dried material, 

 the examination of the species has more generally been confined to the shell alone. 

 Indeed, previous to my account in 1865, only a few of the more common 

 littoral forms had been subjected to an anatomical examination of the enclosed 

 animal, and it was supposed that the structural details in the other Cytheridae 

 were of a quite similar kind. On a carefull examination of the limbs in the 

 various forms observed by me off the Norwegian coast, I found so many 

 essential differences in their structure, that a subdivision of the genus Cythere 

 into several distinct genera appeared to me quite necessary. 14 such genera 

 were of course established, and characterised in my account, both according 

 to the structure of the shell and to that of the limbs. Of these genera 3 had 

 been previously proposed, but only founded on fossil shells, and a 4th genus 



