MOLLUSCA. 57 



be understood. In some genera, as Area {pi. 76, ^g. 32), the teeth are 

 alike, and form a crennlated line along the dorsal margin of the shell. 



In the seventh volume of the Nouv. Mem. de la Soc. Helvet. des Sc. Xat., 

 Professor Agassiz has proposed an imj^roved nomenclature for the armature 

 of the hinge in Yenus and the allied forms, but which may be extended to 

 other families. The annexed diagram rej)resents the relative positions and 



Accessory. . 



Zunular. . . 

 Cardinal. . . 



Ligamentanj. 



Lxiiiular. 



Cardinal. 



Ligarnentary. 



names of the teeth, the upper part representing the anterior part of the shell, 

 its back being towards the observer, and the right valve upon the right 

 hand. The presence of the accessory tooth in the left valve distinguishes 

 the genus Cytherea from Yenus. The same name is applied to a tooth 

 and its corresponding cavity in the opposite valve. 



Most of the Conchifera are marine, living attached, half buried, or 

 entirely buried in the sand or mud, some at the bottom of a perforatic>n 

 from a few inches to several feet deep. Others perforate wood, and some 

 rocks ; and as they increase in size, enlarge the prison which they are 

 never to quit, receiving their nourishment through the opening b}^ which 

 they entered. 



The Conchifera may be divided into several tribes, named liudista^ 

 Tnclusa, and ElatbbrancMa. The first of these, Rudista, is doubtful as a 

 separate section, being composed of bivalve shells, the lower one of which 

 is adherent, and the upper raised in a conical form ; but their true nature 

 and position in the animal scale have not been determined, and they occur 

 only in a fossil state. Some authors jilace them near Chama in the 

 Elatobranchia ; others think them Brachiopoda, and even Tunicata ; and 

 others elevate them to the rank of a distinct order. Some of these bivalves 

 were at one time believed to present the chambered structure of Orthoceras, 

 and they have on this account been incorrectly referred to the Cephalopoda 

 by some authors. This was the case with iripinirites.^ one of the principal 

 genera, which was placed near Belemnites. In Hippurites there is, indeed, 

 an imitation of a chambered structure, but it has an analogy to that 

 observed in certain species of Ostrea, when the shell is sawed in two across 

 the foliated layers, the siphon of the Cephalopoda having no existence. 



• TH})e Inclusa. 



The membei"a of this tribe, although inclosed in bivalve shells, resemble 

 the Tunicata in having the margins of the mantle closed for about three 

 fourths of its circumference. The more typical forms have the shell 

 gaping, and a long double siphon posteriorly, which cannot be entirely 

 withdrawn. 



261 



