Length, 



1 



IT 



Greensand, Warminster. 



Height, 



■ij 



Chalk-marl, Dover. 



Thickness, 



A 



Detritus, Charing. 



Chalk, South-East England, 



THE CRETACEOUS FORMATION. 23 



No. 1. Bairdia subdeltoidea, Milnster. Tab. V, fig. 15 a—f". 



Cythebe subdeltoidea, Miinster. 1830. Jahrbuch f. Min. p. 61, n. 13. 



Cythebina subdeltoidea, Roemer, 1838. Jahrbuch f. Min. p. 517, n. 16, pi. vi, fig. 16. 



— — — 1840. Verstein. Kreid. p. 105, n. 6, pi. 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. 



Tertiary, North Germany {Miinster). 



— France {Miinster). 



— Italy {Miinster). 



— Valparaiso (?), South America. 



Chalk Formation, Bohemia {Reuss). Eocene, Hauteville, Normandy. 



Chalk, Maestricht {Miinster and Bosquet). — Lower Fresh- water Formation, Isle of Wight. 



— North Germany {Miinster and Rctmer). Miocene, Virginia, North America. 



— Weinbohla. Phocene, Coralline Crag, Sutton^ and Walton. 



— Royan, South France. 



Recent. 



Australia, Sydney, finely punctate. 

 Bahama, Froyidence, 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 rigid 

 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 Sc)rthian 

 bow, arched at its anterior and posterior thirds, and incurved at the middle ; 

 strongly beaked posteriorly ; subacute anteriorly. Dorsal edge slightly inverted at its 



1 In Mr. S. Wood's Collection. 2 j^ ]y[]-. Williamson's Collection. 



' 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 fossD. individuals. 



