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 I'ight (smaller) valve are trenchant and bevilled off 

 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 by M. Cornuel " la lame 

 pectorale." These laminae 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 Candonce by the Cretaceous species Ci/there Hilseaua, Roem. In the Candonse 

 referred to the hinge-bars are simple and not produced at their extremities into teeth, 

 but merely " knurled" throughout their length : in C. liilseana 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 liilseana, the contact-margin of the right valve has a flange, 

 which is received into a corresponding gi'oove 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 CythereUa ; the groove and 

 flange, however, in the latter, are much more distinct 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 



