CYTHERIDEA. 17^ 



1868. Cytheridea toeosa, Bradij. Monog. Rec. Brit. Ostrac, p. 425, pi. xxviii, figs. 



7 — 12 ; pi. xxxix, fig. 5. 

 1868. — LiTTOBALis, Brady. Nat. Hist. Trans. Northumb. and Durham, 



vol. iii, p. 125. 

 1870. — TOROSA, var. teres, Brady and Robertson. Ann. and Mag. 



N. H., ser. 4, 

 vol. vii, p. 21. 



This differs from the typical form in being altogether destitute of tubercular pro- 

 minences, which are replaced in most cases by a shallow transverse median sulcus on the 

 lateral aspect of the valves. 



C. torosa is usually an inhabitant of brackish water, where it is often found living in 

 immense numbers, the situations which it haunts being chiefly salt marshes and muddy 

 estuaries. It has, however, though very rarely, been found in quite fresh water, and 

 more frequently in shallow littoral situations exposed to purely marine influences ; but 

 in neither of these latter cases does it ever occur Very abundantly. 



We have not oui'selves met with the typical tuberculated form of this species amongst 

 Post-tertiary fossils, except in one locality, our examples being otherwise referable to the 

 smooth variety {teres, Brady and Robertson). 



Distribution. Becent. — Norway, Great Britain, Ireland, Levant, Sea of Azoff. 



Fossil. — England : Grays Thurrock. Scotland : Drip Bridge, Raised beach at Oban. 

 South Wales : New Dock basin at Cardiff. Ireland : Portrush. 



4. Cytheridea lacustris {G. 0. JSars). Plate VI, figs. 16 — 20. 



1863. Cytheee [lacustris, G. O. Sars. Om en i Sommeren 1862 foretagen 



zoologisk Reise, p. 30. 

 1868. Cytheridea — Brady. Monog. Rec. Brit. Ostrac, p. 427, pi. xxvi, 



figs. 18—21 ; pi. xl, fig. 2. 



Carapace of the female, as seen from the side, subquadrangular, highest at the 

 anterior third, greatest height equal to considerably more than half the length ; anterior 

 extremity broad and well rounded ; posterior narrow and not so boldly rounded ; 

 superior margin highest over the anterior hinge, thence sloping rather steeply and in a 

 more or less undulating line backwards ; inferior margin deeply sinuated in the middle, 

 thence sloping upwards and backwards with a gentle curve. Outline, as seen from above, 

 ovate, the lateral margins subparallel, irregularly sulcate and tuberculate, extremities 

 obtusely pointed, greatest width equal to half the length. End-view broadly and irregularly 

 ovate. Surface of the shell marked with closely set angular pittings, and bearing on the 

 lateral aspect several (2 — 5) large rounded tubercles. Hinge-processes (fig. 20) not 



