CLASS I PELECY PODA 357 
valves by calcification deposited about it, it occupies a depression in the 
cardinal margin which Bernard has regarded as internal. In a sense it is 
internal, but its position at this stage is not significant, and there is no 
fundamental difference between the cases. The differentiation in function 
and structure which we find in the adult between the ligament, properly 
so-called, and the “internal ligament ” or resilium, is a later development. 
The ligament may be regarded as a fundamental character of Pelecypods, 
and is universally present, though in some cases as a mere degraded rudiment 
(Pholadacea) ; it may be separated from the valves and functionless (Chlamy- 
doconcha), or present only in the young stages and lost through specialisation 
due to the sessile habit (fudistae). 
As the most important factor in the mechanism of the valves, the liga- 
ment has undoubtedly developed with the evolution of the class, and its 
chief modifications date from the earliest period in the life-history of the 
group. The function of the original ligament was that of an external link 
between the valves having the essential nature of a C-spring. That is, the 
insertion of the ligament edges on the cardinal margins, or, at a later period, 
on thickened ridges or nymphae by which these margins are reinforced to bear 
strains, resulted in the following conditions :—The valves being held together 
and, in closing, approximated by the contraction of the adductor muscles, the 
preservation of their precise apposition, marginally, is due to a rotary motion, 
exerted along the axis of the ligament, which pulls the attached edges of the 
ligament nearer to each other and exerts a strain on its cylindrical exterior. 
This operation, with a thin ligament, involves a tensile strain on the whole 
cylinder; with a thick ligament the external layers are strained and the 
internal layers compressed, so that, to the tensile elasticity of the external 
layers is added the compressional elasticity of the internal portion. The 
result of the differing strains to which the several layers of the ligament are 
subjected brings about a difference of structure, and, whenever the ligament 
becomes deep-seated, there is a tendency for the respective parts to separate 
along the line where the two sets of strains approximate. We then have two 
elastic bodies, operating reciprocally in opposite directions, the outer or liga- 
ment proper tending to pull the valves open to a certain distance correspond- 
ing to its range of tensional elasticity ; and the other or resiliwm (for which 
the objectionable terms “cartilage” and “internal ligament ” have been used) 
tending to push them open to an extent corresponding to its range of ex- 
pansion. 
The ligament proper is of a more or less horny nature, tough, and semi- 
translucent beneath its external surface. When dry it has a vitreous fracture, 
and often shows hardly any fibrous texture. 
The resilium is distinctly lamellar or composed of horny fibres, which are 
apt to give a pearly sheen to its broken surface. There is often a more or 
less extensive intermixture of lime in its substance, which may be diffused, or 
may be especially concentrated along the median plane. As may be seen by 
examining the unbroken resilium (as in JJactra), this organ in such cases has 
something of an hour-glass shape ; the ends which fit into the “cartilage pits ” 
or resilifers being more expanded than the centre between them. The deposit 
of lime in the form of an accessory shelly piece, usually termed the ossiculum 
or lithodesma, serves for the reinforcement of the resilium. 
For the type of ligament which extends on both sides of the beaks, 
