THE TERTIARY FORMATION. 41 



of small tubercles, forming a knurling or fine crenulation on this edge, somewhat like 

 the hinge-teeth of Nucula, the other valve having corresponding pits on its dorsal 

 margin : these rows of small hinge-teeth are either continuous along the hinge-margin 

 (as in Dana's figure of Cythere Americana), or are discontinued (to a greater or less 

 extent) in the central part and form isolated groups at the anterior and posterior 

 angles of the dorsal margin ; whilst the intermediate portion of the margin has still 

 smaller tubercles on the left, and corresponding pittings on the right valve 1 (this is 

 also the case in Cyprideis \ Cythere ?~\ torosa) : ventral margins somewhat incurved. 



No. 1. Cytheridea Mulleri, Munster, sp. Plate V, figs. 4 a — 4 c, and 5; PI. VI, 



figs. 10 a, [Ob, and 11 — 13. 



Cythere Mulleri, Munster. Jahrb. f. Min., &c, 1830, p. 02; and Neues Jahrb., &c, 



1835, p. 446. 

 Cytherina Mulleri, Roemer. Ibid., 1838, p. 516, t. 6, fig. 6. 



— — Reuss. Haidinger's Abhandl., iii, p. 55, t. 8, fig. 21. 



— intermedia, Reuss. Ibid., p. 86, t. 11, fig. 12. [C. Mulleri, var.] 

 Cytheridea Mulleri, Bosquet. Mem. Couron. Acad. Belg., xxiv, p. 39, t. 2, fig. 4. 



— — Jones. Quart. Journ. Geol. Soc, x, p. 160, t. 3, fig. 7; and Mem. 



Geol. Survey, 1856, p. 158, t. 7, fig. 28. 



INCH. 



Length, -^ Recent : Holland ; Australia. 



Tertiary : England ; Europe ; Australia 1 



Carapace ovately triangular, rounded anteriorly, obliquely pointed behind; dorsal 

 margin arched ; ventral margin straight or slightly incurved ; anterior margin spinous : 

 surface coarsely punctate ; the pits generally associated with semicircular furrows on 

 the fore part of the valves, and with longitudinal furrows on the ventral part ; on the 

 posterior half of the valve the pittings follow transverse sunken lines, parallel with the 

 transverse sulcus which is more or less apparent on the centre of each valve, at the 

 place of the lucid spots. 



Anterior to this median furrow frequently occurs a tubercle (pi. 6, fig, 11), or at 

 least a marked elevation of the surface (pi. 6, fig. 10 a). In some specimens from the 

 Barton Clay, however, the furrow and tubercle are absent (pi. 5, fig. 4 a). 



Lucid spots small, four, arranged in a transverse row (System c, see page 6). 



Dorsal profile narrow oblong, very slightly incurved at the sides, and obtusely 

 angular at the ends ; end-view ovate. 



1 This form of hingement is also apparent in Zenker's figure of the hinge-line of the species which Ju- 

 lias described as Cythere viridis, ' Archiv f. Natur.,' loc. cit. t. 4, fig. 10. 



6 



