THE CRETACEOUS FORMATION. 
9 
peach-stone. The valves are irregularly oval and gibbous, spinous at their anterior 
and posterior borders, and generally marked by punctations or by reticulated wrinkles. 
The contact-margin of the dorsal border of each valve has, occupying its central third, 
a longitudinal ridge or bar, and an accompanying furrow. In the right valve the ridge 
is next to the outer edge, and the furrow lies within the ridge ; in the opposite valve 
the furrow lies just within the edge of the margin and the ridge on the inner side of 
the furrow ; consequently, the ridge or bar of one valve fits the furrow of the other, and 
vice versa. The bars are narrow, rounded, polished, and finely crenulate or “knurled.” 
The anterior’ and posterior extremities of these bars afford processes or teeth, forming 
the anterior and posterior hinges of the carapace. In the right valve the hinge-teeth 
are prominent, and, owing to the proximity of the bar to the outer border, they some- 
times appear to form part of the outer edge of the shell. On their inner or lower side 
are placed cavities for the reception of the hinge-teeth of the opposite valve. In the 
left valve the teeth are less strongly developed, especially at the posterior extremity 
of the hinge-bar, and have the accompanying tooth-sockets and the long furrow on 
their outside, separating them from the outer edge of the shell. The anterior, ventral, 
and posterior margins of the right (smaller) valve are trenchant and bevilled oft' 
internally, and lie within the similarly formed but overlapping edges of the left (larger) 
valve, a slight groove or ledge for their reception being generally apparent. The 
middle of the ventral border of each valve is somewhat incurved, and bears a thin 
semilunar process projecting from the contact-margin, termed byM. Cornuel “la lame 
pectorale.” These laminse are formed by a local increase of the outermost or free edge 
of the margins, and in the closed carapace the lamina of the smaller valve lies 
within that of the larger, a slight cavity or sinus being provided for its reception. 
This section is connected in the form of shell and mode of hingement with certain 
recent Candonoi by the Cretaceous species Cythere Hilseana, Roem. In the Candonae 
referred to the hinge-bars are simple and not produced at their extremities into teeth, 
but merely “ knurled” throughout their length ; in C. Hilseana the bars are but slightly 
modified, the hinge-bar of the right valve being somewhat thickened at its anterior and 
posterior extremities, and marked with three or four knurlings stronger than those on 
the rest of the ridge, the opposite valve having cavities to receive them. In these 
Candonse, and in Cythere Hilseana, the contact-margin of the right valve has a flange, 
which is received into a corresponding groove in the larger valve. In other species of 
Cythere proper the groove and flange become nearly obsolete. A mode of contact, 
similar to the above, obtains in our sub-genera Cythereis and Cytherella ; the groove and 
flange, however, in the latter, are much more distinet and uniform along the contact- 
margins ; and, moreover, in Cytherella it is the right valve that is grooved, and 
the left that is flanged, the contrary condition to that which exists in Cythere and 
Cythereis. 
The species belonging to this section that occur in the Cretaceous strata are not 
c 
