THE TERTIARY EORMATION. 43 



valves raised up into irregular lumps or bosses. Of these knobs, which are often but 

 ill defined, sometimes seven can be counted on one valve. The spots most usually 

 occupied, when the bosses are but few, are the posterior part of the valve and 

 especially the central part immediately in advance of the sulcus and the place of the 

 lucid spots. In its general form C. torosa resembles that variety of C. Mutteri in 

 which the posterior extremity is contracted and acuminate (var. acuminata, 

 Bosquet). 



This torose variety was found plentifully by Mr. Rosser in the same beds of the 

 Woolwich series as yielded the typical C. Mutteri. It occurs also abundantly in some 

 of the shales of the Hempstead series, in Hempstead Cliff, Isle of Wight. 



No. 2. Cytheridea debtlts, spec. nov. Plate V, fig. 5 ; PI. VI, fig. 13. 



INCH. 



Length, -fa Recent : Norway. 



Upper Eocene : Isle of Wight. 



This is a small form, nearly resembling C. Mutteri in outline, but much smaller, 

 and more obtuse ; not so acute posteriorly as C. Mutteri, var. torosa ; often destitute of 

 the median impression, and bearing a weakly marked punctation. It is sometimes, 

 however, larger than usual, and more strongly punctate. 



Cythere [Cytheridea) debilis occurs with the typical C. Mutteri in numbers in an 

 Oyster-band, a Nucula-bed, 1 and other deposits, of the Upper Eocene series at 

 Colwell Bay, Isle of Wight. A similar form occurs recent on the coast of Norway. 



No. 3. Cytheridea pinguis, spec. nov. Plate II, figs. 4 a — 4h. 



INCH. 



Length, -Jj- Pliocene : Suffolk. 



Carapace thick, ovate-triangular; sub-cylindrical when young; lowest behind; 

 highest at the anterior third, where it is also somewhat compressed ; dorsal border 

 more or less arched ; ventral border straight, with its edge somewhat inturned : 

 surface of valves in the young state finely punctate and beset with scattered papilla) 

 (fig. 4g), in older individuals coarsely punctate (fig. 4 h) : hinge well developed. 



Dorsal aspect sub-oblong ; end-view sub-orbicular. 



Cythere {Cytheridea) pinguis is nearly related to C. Mutteri on one side and 



1 The specimens from this bed were kindly communicated to me by my friend Mr. Harris, of Charing. 



