MOLLUSCA. 57 
be understood. In some genera, as Arca (pil. 76, fig. 32), the teeth are 
alike, and form a crenulated line along the dorsal margin of the shell. 
In the seventh volume of the Nouv. Mém. de la Soc. Helvét. des Sc. Nat., 
Professor Agassiz has proposed an improved nomenclature for the armature 
of the hinge in Venus and the allied forms, but which may be extended to 
other families. The annexed diagram represents the relative positions and 
‘ane 
\ | 
Accessory. 
Tunular. . . Sener 
Cardinal... Cardinal. 
Ligamentary. Ligamentary. 
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 Venus. 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 perforation 
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 by which 
they entered. 
The Conchifera may be divided into several tribes, named S?udista, 
Inclusa, and Elatobranchia. 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 place 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 AZippurites, 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 acruss 
the foliated layers, the siphon of the Cephalopoda having no existence. 
Tribe Inclusa. 
The members 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 
