88 MR. GARNER ON THE ANATOMY OF THE 
Conchifera, I would observe that it can only happen when there is an erosion or giving 
way of the cartilage, as the beaks, by means of the cartilage, are always originally in 
connexion. In Jsocardia, for instance, we see the beaks remote and the valves volute ; 
and it may be seen that as the cartilage increases behind, it gives way and becomes bi- 
furcate before. Sometimes this giving way takes place more from one valve than from 
the other, in which case one valve may be much longer in its beak than the other, as is 
seen in Gryphea, &c. We cannot account for the growth of some shells, without ad- 
mitting that the animal has the power of lessening them at certain points ; thus in the 
under valve of the Anomia, how can we explain the enlargement of the notch, an almost 
perfect foramen, without admitting such a power, which appears exerted through the 
influence of the constant currents of water produced by the vibratile cilia of the animal ? 
The coloration of the valves, of the foot, of the syphons, &c. is produced by a secreted 
colouring matter, the chemical nature of which is not known, but a similar secretion to 
which is found in all Mollusca. This secretion has the property of taking a much 
brighter tint on exposure to light’. It is secreted either from the veins or from 
venous secreting organs, to be hereafter mentioned*. The markings of the valves 
appear to be caused by the disposition of the veins of the mantle, and are often inter- 
rupted, from the cessation of the secretion at certain periods. 
The varieties in the articulation of the valves, in the cartilage, ligament, and teeth, 
may be thus explained. When, as in the Cardium, the cartilage is external, and con- 
vex and prominent above, its compression does not happen from the pressure of the 
valves, as is the case with the species possessing internal cartilages, but from its bend- 
ing upon itself. The cartilage of this conformation differs from that of the other by 
its containing a portion of carbonate of lime in its composition. The teeth are wanting 
or weak when there is great strength of muscle or cartilage ; when the irregularity of 
the edge of the valves prevents sliding motion; or when the shell is small, flat, and 
polished, and hence little exposed to violence; also when the hinge and cartilage are 
long ; though they are numerous in the long hinge of the Arcacea, compensating for the 
weakness of the cartilage. We see a distinct pinnate process of the mantle for the pur- 
pose of secreting the numerous teeth of the Nucula. In those genera which have long 
fleshy siphons and gaping shells, the cartilage is internal, and situated upon a project- 
ing process of one of the valves, as in the Mya, by which disposition the shell is not 
readily quite closed, nor far opened. The shell is only allowed to be opened widely 
when the lobes of the mantle are conjoined to a small extent, as is done by the internal 
cartilage of many of the Monomyaria. When, as in the Arca, the foot is thick, we see 
in the straight hnear hinge, and in the remoteness of the beaks, a provision for the con- 
‘ To produce the famous purple of the ancients, it appears only necessary to expose the soft parts of the 
animal of the Purpura to the influence of the light and air, when its natural brownish secretion becomes of a 
bright purple colour. 
* And not from the liver, as Blainville says. 
