RECENT BRITISH OSTRACODA. 



357 



name of the genus {Polijcope) is founded. On account of the minuteness of the animals, 

 Sars has not heen able to observe accurately their internal organization ; but he asserts 

 that the eyes are entirely wanting, and that the heart also appears to be absent. The 

 digestive canal has a single capacious stomach, as in the foregoing section. 



4. Platycopa. — This group is typified by the genus Cytherella, known, before Sars's 

 researches, only from fossil specimens. The lower antennae exhibit a structure so peculiar 

 that this character alone would amj)ly separate them from the other Ostracoda. They arc 

 very strongly developed, bearing considerable likeness to the swimming-feet of the 

 Copepoda, but are still more muscular, and constitute very powerful swimming-organs. 

 These consist of a broad flattened basal portion and two biarticulate broad branches, which 

 bear numerous long bristles as in the Copepoda. The upper antennse are of exceedingly 

 robust build, and appear also to serve as locomotive organs. The mandibles are small 

 and weak ; but their palps are strongly developed, and that of the first jaw is armed 

 internally with an elevated ridge which is beset with numerous toothed and flexuous 

 setae, bearing a remarkable likeness to the feet of the Cladocera [Sida, Davhnia). Behind 

 the mouth follow, besides the first pair of jaws, only two pairs of limbs, of which neither 

 has the remotest resemblance to feet ; the last is, indeed, in the female, quite rudi- 

 mentary, forming only a small setose lamina. Each of the two preceding pairs bears at 

 the base a large halfmoon-shaped, numerously ciliated branchial plate. The abdomen is 

 divided into several segments, most of which bear on the hinder part fasciculi of bristles. 

 The postabdomen consists of two small plates, which are separate through their entire 

 length, and bear at the extremity a number of slender spines. The shell is of very 

 compact structure, so that it is found almost unaltered in very early geological forma- 

 tions. This group contains only one genus, Cytherella. 



I have endeavoured to give, under each marine species, as accurate a statement of its 

 distribution, both in the recent and fossil state, as the present state of our knowledge 

 admits of ; and with the view of indicating more clearly the relations of the species, I 

 have classified them under several types, according to the geograpliical areas which 

 they chiefly inhabit, the typical designation being always taken from the region in which 

 the species appears at the present time to be most abundant. 



We have at present scarcely suificient knowledge of the distribution of the freshwater 

 species to render a similar attempt with regard to them desirable. 



The typical areas here adopted are as follows : — 



1. Arctic type, including all species attaining their greatest development north of the 



isothermal line of 32° Pahr, Glacial might perhaps have been a more appropriate 

 term for this group, but would have been liable to ])e confounded with the post- 

 tertiary geological formations which go under the same name. My very limited 

 knowledge of species from this tract is derived from specimens dredged by Dr. 

 Sutherland, and from some parcels of mud obtained from whaling-ships by D. Ro- 

 bertson, Esq., of Glasgow. 



2. Scandinavian type, comprehending the seas of Iceland, Norway, Sweden, Denmark, 



Shetland, and Northern Scotland. The Ostracoda of this district have been dili- 

 gently collected by many naturalists, especially by Miiller, Lilljeborg, G. O. Sars, 



