THE CRETACEOUS FORMATION. 
15 
On the valves three eminences or tubercles are more or less strongly developed ; 
one rather anterior to the centre, which is very characteristic of this section, as 
pointed out by M. Bosquet (Op. cit., p. 10), and one at each angle formed by the 
junction of the posterior with the superior and inferior borders. From each of these 
last-mentioned tubercles a ridge generally arises, which is continued more or less un- 
interruptedly around the edge of the valve. This is characteristic of the Margined^' 
of Dr. Reuss, but it is a character common also to the “ Cornidcs” and found amongst 
the Cytlierellce, nob. The hinges are formed of the same elements as in Cythere 
proper, but the bars are scarcely distinct from the margins of the valves and the 
furrows are nearly obsolete ; the hinge-teeth, however, and the sockets for their 
reception are more strongly developed than in Cythere. The hinge-margin, is 
proportionally longer than in Cythere proper, and consists of the straight dorsal edge, 
extending from the anterior to the posterior hinges, which respectively occupy the 
angles formed by the junction of the dorsal with the anterior and posterior borders. 
The hinge-margin of the right valve is narrow and trenchant ; its anterior extremity is 
suddenly produced into a conical tooth, having a slight cavity on its inner aspect ; the 
posterior extremity affords a smaller process, of an oblong shape, accompanied by a 
slight cavity similar to the above. In the left valve the hinge-margin bears a slight 
gi'oove along its surface of contact, and at each of its extremities is a semicircular or 
horseshoe-shaped cavity or socket for the reception of the hinge-teeth of the opposite 
valve. These cavities are excavated in the valve-margin, which is thickened and 
projects outwards at the anterior and posterior angles of the dorsal border. The pro- 
jection at the anterior hinge is the strongest, where it forms a semicircular process or 
ear-like appendage, which is peculiar to this section, as noticed by M. Bosquet (Op. 
cit., p. 10). Externally this process bears a tubercle corresponding to the cavity, and 
the summit of the tubercle is occupied by a small, highly polished, translucent bead. 
The anterior extremity of the hinge-margin is produced into a slight conical process, 
and forms the posterior boundar}^ of the anterior socket ; this socket is bounded in 
front by a narrow process running downwards at a right angle from the inner edge of 
the valve-margin, and terminating in a small conical tooth ; so that this socket is 
accompanied by two small teeth, which, when the carapace is closed, fit to the inner 
or under side of the cardinal tooth of the right valve. The posterior extremity of the 
hinge-margin forms the anterior boundary to the posterior socket, and is furnished with 
a slight process, but this socket and its accompanying teeth are much less strongly 
developed than the similar parts of the anterior hinge. 
The other margins of contact have an arrangement very similar to that in Cythere 
proper, except that the flange of the right and the groove of the left valve are 
generally more distinct, and the ventral laminae more strongly developed. 
This subgenus is more plentiful in the Cretaceous Formation than in the Tertiary 
deposits, or in the recent state. 
