A Synopsis of the Recent British Ostracoda. Ill 



almost incomprehensible, some strata of certain rock formations 

 seeming to be almost entirely composed of them. Amongst 

 the recent species I know of no case analogous to this, except 

 that of the dwellers in salt marshes and estuary mud ; and I 

 have no doubt that, were the mud-banks of our tidal rivers, 

 and the swamps adjacent, suddenly petrified, we should, in 

 many cases, find that the resulting stratified rock would exhibit 

 as wonderful a concourse of once-living crustacean shells, as 

 those which have just been spoken of. Mr. W. K. Parker has 

 indeed conjectured, from a study of their fossil Bhizopoda, 

 that the post-tertiary clays of the district round Peterborough 

 constitute a littoral, brackish water-deposit ; and it is remark- 

 able that the Foraminifera there found, agree, to a very large 

 extent, with those which I have myself taken in the salt 

 marshes of our north-eastern coast. In strata much older 

 than the post-tertiary, one would not, of course, expect to find 

 species exactly identical with those now living. In washings 

 of these clays, however, with which Mr. Parker has kindly 

 supplied me, I have not been able to detect any shells of 

 Ostracoda; it is, indeed, noticeable, that fossiliferous strata 

 which are rich in Foraminifera are mostly poor in Ostracoda, 

 and vice versa. Such, on consideration, one would naturally 

 expect to be often the case. In all the salt marshes which I 

 have had the opportunity of examining, living Ostracoda 

 have been very abundant. If the net be passed carefully 

 along the surface of the soft mud, so as to take up a thin 

 layer, and the mass thus obtained be then washed through 

 the net sufficiently to clear it of the fine ooze which will be 

 found to constitute the greater part of its bulk, the residue 

 will mostly consist of Ostracoda, living and dead, grains of 

 sand, fragments of decaying vegetable matter, and very pro- 

 bably a good many living Foraminifera. This will mostly 

 be the result wherever the water is brackish, and deposits a 

 good deal of slimy mud, but in fresh water, or at the mouths 

 of rivers, where the stream is clear and rapid, and does not 

 produce much fine deposit, the Ostracoda, aud other Microzoa, 

 will be found much less plentifully. My belief is, therefore, 

 that those strata which exhibit such very abundant and closely- 

 packed remains of the smaller Cypridas and Cytheridas have 

 most likely been formed in shallow, brackish lagoons, or at the 

 mouths and deltas of rivers. The species of Ostracoda which 

 I have found in these situations are, Oytheridea torosa (Jones), 

 Oy there pellucida Baird, and Loxoconcha elliptica Brady ; 

 while in water, a little further from the saline influence, but 

 still slightly partaking of it, it is not uncommon to meet with 

 Oypris salina Brady, and Cypridopsis aculeata (Lilljeborg), as 

 well as Entomostraca belonging to other orders. Some 



