THE CRETACEOUS FORMATION. 
27 
centre ; a slight furrow lies between each of the little ridges and the outer margin 
(fig. 18 f). The right valve is provided with a strong ventral lamina (fig. 18 e). 
Dorsal aspect elongate oval ; anterior varying from ovate to oval. 
Munster’s description of C. angusta is quite applicable to this species, and Rocmer’s 
C. lavigata is not materially different. Munster 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. amggdalina, M‘Coy (Syn. Charact. &c. pi. xxiii, fig. 8), does not appear to differ 
essentially from this species. 
No. 5. Baird I A triquetra, nohis. Tab. VI, fig. 19 a — c. 
INCH. 
Length, J3- Greensand, Black down. 
Height, P-r- Chalk, Gravesend. 
Thickness, gL 
Carapace-valve (left) triangular, convex, smooth. Dorsal margin strongly arched ; 
ventral straight. Anterior extremitg rounded ; jjosterior subacute, rather narrower than 
the anterior. Seen in profile, the posterior extremity of the valve is more obtuse than 
the anterior. 
This species, which slightly resembles C. arcuata, Munster (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, tiobis. Tab. VI, fig. 20 a — c. 
INCH. 
Length, 
Height, gL Detritus, Charing. 
Thickness, gL 
Carapace-valve (left) somewhat oblong, convex, smooth. Anterior extremitg 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 of B. Silicula. 
