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 " Marginatm' 

 of Dr. Reuss, but it is a character common also to the " Cornutm" and found amongst 

 the CytherellcB, nob. The hinges are formed of the same elements as in Cytliere 

 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 CytUere. The hinge-margin, is 

 proportionally longer than in Cytliere 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 riyJd 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 boundary 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. 



