THE CRETACEOUS FORMATION. 
23 
No. 1. Bairdia subdeltoidea, Munster. Tab. V, fig. 15 a—f". 
Cytheke subdeltoidea, Munster. 1830. Jalirbuch f. Min. p. 61, n. 13. 
Cytherina subdeltoidea, Rcemer, 1838. Jahrbucli f. Min. p. 517, n. 16, pi. vi, fig. 16. 
— — — 1840. Verstein. Ki’eid. p. 105, n. 6, pL xvi, fig. 22. 
— — Reuss. 1845. Verstein. Bohm. Kreid. p. 16, n. 1, pi. v, fig. 38. 
— TKIGONA, Bosquet. 1847. Ent. foss. Maestricht, p. 8, n. 3. pi. i, fig. 3 a — e. 
INCH. 
Length, Greensand, Warminster. Tertiary, North Germany (ilfawsto-). 
Height, Chalk-marl, Dover. — France (Miinster). 
Thickness, Detritus, Charing. — Italy {Miinster). 
Chalk, South-East England. — Valparaiso (?), South America, 
Chalk Formation, Bohemia (Reuss). 
Chalk, Maestricht {Miinster and Bosquet). 
— North Germany {Miinster and Rcemer). 
— Weinbohla. 
— Royan, South France. 
Eocene, Hauteville, Normandy. 
— Lower Fresh- water Formation, Isle of Wight 
Miocene, Virginia, North America. 
Pliocene, Coralline Crag, Sutton^ and Walton. 
Recent. 
Australia, Sydney, finely punctate. 
Bahama, Providence, finely punctate. 
— — finely punctate and hairy. 
— — smooth. 
— Turk’s Island, finely punctate. 
Mauritius, finely punctate. 
Manilla, finely punctate and hairy.^ 
North Britain, Arran, narrow variety, finely punctate. 
Carapace triangular, resembling a thick orange-pip. Valves strongly convex, 
generally smooth, sometimes slightly-spined/ and shining; extremities sometimes 
spined. Left (large) valve protruding and somewhat angular on the dorsal, elliptical 
on the ventral margin ; beaked posteriorly, obtuse anteriorly ; dorsal edge inverted 
nearly the whole of its length ; ventral edge inverted near the middle. In the right 
valve, which is narrower than the left, the projecting dorsal border is truncated, 
forming three sides of a hexagon ; the ventral border is sinuous, shaped like that of 
the opposite valve, except that it is compressed at the centre, resembling a Scythian 
bow, arched at its anterior and posterior thirds, and incurved at the middle ; 
strongly beaked posteriorly ; subacute anteriorly. Dorsal edge slightly inverted at its 
^ In Mr. S. Wood’s Collection. ^ In Mr. Williamson’s Collection. 
3 It is not improbable that the surface of the valves originally bore punctations, now defaced, as is the 
case with Cytherella truncata, the Chalk specimens of which are plain, whilst many of the better preserved 
specimens from the Gault exhibit pittings. The recent specimens are in general finely punctated ; at Provi- 
dence, however, there occur individuals without pittings, and at the same place specimens occur which are 
both hirsute and punctate, a condition probably frequent in living individuals : through Mr. Williamson’s 
kindness in lending me his collection of recent Entomostraca for comparison, I have seen individuals from 
Manilla similarly characterised to the last mentioned,, and specimens from Tenedos bearing marginal spines, 
traces of which condition exist in a few fossil individuals. 
