6 KOCENE MOLLUSCA. 
in the fossil state. ‘This is: also exceedingly variable, being thick and opaque in some 
species, while in others it is thin and transparent, and in many glossy and polished 
shells itis entirely absent. ‘The epidermis is said to be a protection to the shell against the 
chemical action of acids held in solution by the water; but many fresh- and salt-water 
species, although covered with a thick and woolly epidermis, suffer materially by the erosion 
of the shell, while those that are less protected by this covering appear to escape altogether. 
It is at the umbones, or that. part of the shell which is most remote from the more vital 
portions of the animal, and in those species which appear to contain a less quantity of 
animal matter, that the ravages are committed. In some glossy and polished shells, 
such as in the Venus tribe, there is a total absence of epidermis, and these do not. suffer 
from erosion. 
The two valves are held together by an elastic ligature: this has been called 
ligament when it is situated on the external part of the shell, and cartilage when it 
is internal; but as this fastening is sometimes composed of a portion of each, we are on 
those occasions unable to characterise it by either of those terms a/oxe, and I think it ought 
to have a peculiar name for its joint and general service ; I propose, therefore, to call it 
connexus (the comector). When this ligature is external, it acts over a support or fulcrum, 
and by its elasticity contracts in order to open the valves; it is then Zigamentous. When it 
is placed within the outer margin of the shell, it is carti/aginous, opening the valves by its 
expansion; and when it partakes the double character of ligament and cartilage, it is 
aimphidesmous oy bipartite. This elastic material is then extended both outwardly and 
inwardly, and although in this case the ligament and cartilage are united, there is always 
a slight elevation or partition between them, and this forms the fulcrum over which the 
ligament acts. The two pieces of the Bivalve are thus naturally kept open by the 
elasticity of the ligamentous portion in its contraction, and by the cartilaginous in its 
expansion, and when the two valves are closed in the living animal it is by the contrac- 
tion of the adductor muscles. In the fossil state Bivalves are of course often found in 
their natural position, that is, with the margin of the valves separated in the way the 
dying animal has left them on the relaxation or rather paralysis of the muscles, 
but the two valves are as often quite closed; this, of course, must be from external 
pressure overcoming the action of the connexus. 
Although the two valves are kept together and held in position by the connexus, they 
are often further secured by prominent processes, called teeth, which prevent a lateral 
motion ; these are said to be strong and prominent where there is weakness in the connexus, 
and, as a general rule, amongst the Dimyaria the observation is more applicable to those 
shells where it is on the outside (7. e. ligamentous), while in Mya, Anatina, &e., which have 
the connexus internal (7. e. cartilaginous), there are no prominences or teeth. This rule, 
however, like most others, is not without exceptions ; Wucula, Leda, Mactra, &c., have the 
dorsal margins furnished with numerous and prominent interlocking teeth ; perhaps, as a 
general rule, the more locomotive species have the strongest hinge. 'The dental apparatus, in 
