THE CRETACEOUS FORMATION. 27 



centre ; a slight furrow lies between each of the little ridges and the outer margin 

 (fig. 18/'). The right valve is provided with a strong ventral lamina (fig. 18 e). 



Dorsal aspect elongate oval ; anterior varying from ovate to oval. 



Miinster's description of C. angusta is quite applicable to this species, and Roemer's 

 C. lavigata is not materially different. Miinster found this species living in the 

 Adriatic, and we have specimens from the British seas. The specimens from the 

 Chalk and Gault have generally a much narrower form than those from the Detritus, 

 which latter, like the recent and some of the Gault specimens, are broader and more 

 convex : when very thick, this species approaches the form of C. aurantia, Baird. 

 C. amygdalina, M'Coy (Syn. Charact. &c. pi. xxiii, fig. 8), does not appear to differ 

 essentially from this species. 



No. .5. Bairdia triquetra, nobis. Tab. VI, fig. 19 a — c. 



INCH. 



Length, -i-^ Greensand, Black down. 



Height, ^'j Chalk, Gravesend. 



Thickness, ^V 



Carapace-valve (left) triangular, convex, smooth. Dorsal margin strongly arched ; 

 ventral straight. Anterior extremity rounded ; postmor subacute, rather narrower than 

 the anterior. Seen in profile, the posterior extremity of the valve is more obtuse than 

 the anterior. 



This species, which shghtly resembles C. arcuata, Miinster (Jahrb. 1838, pi. vi, 

 fig. 17), is more triangular than B. angusta; the latter is higher at the anterior and 

 posterior thirds, also thicker posteriorly, and less convex centrally, than B. triquetra. 



In the Chalk of Gravesend we have found two separate valves only of this species, 

 and one valve in the Greensand of Blackdown. 



No. 6. Bairdia Silicula, yiohis. Tab. VI, fig. 20 a — c. 



INCH. 



Length, ^^ 



Height, Jy Detritus, Charing. 



Thickness, -^-^ 



Carapace-valve (left) somewhat oblong, convex, smooth. Anterior extremity semi- 

 circular; posterior oblique, subacute at its inferior angle. Dorsal border somewhat 

 arched ; ventral straight, inverted at its middle. 



Dorsal aspect of the closed carapace elongate oval ; anterior oval. 



We have met with this single valve only oiB. Silicula. 



