STRUCTURE OF SHELL IN BIVALVES. 
37 
placed there; the opposite end forms the Anterior or Cephalic 
Margin, and is the end where the month of the animal is placed. 
The two valves are termed right or left, according as they are placed 
on the right or the left side of the animal, and are easily identified 
and discriminated ; one method is to place the shell in its natural 
position as when the animal is crawling, with the ligament upwards 
and towards the observer, and the opposite or anterior end pointing 
forwards, in this position the right and left valves correspond to the 
right and left hand of the person e-vamining it; or the separate valves 
can be distinguished by placing or holding the valve before you, with 
the inner surface upwards and the ventral margin toward yon, if when 
in this position the ligament lies to the right, then the valve is the 
right valve, if to the left, the left valve ; the functional ligament 
always being more or le.ss posterior to the nmbones, never anterior 
to them. Linn6 and many of the older naturalists described the 
ventral or lower margin of the shell as the top, the hinge-line as the 
base, and the left valve as the right, and vice versa. C. Pfeiffer and 
others follow Draparnaud in regarding the siphonal end as the 
anterior one, and as they consider the ventral margin to be the lower 
one, the right and left valves of these authors necessarily correspond 
with the left and right valves respectively of English conchologists. 
The shells of the Uniovido', are like those of man}' Gastropods 
composed of three distinct layers; the outer layer or epicouch covers 
the external surface, and is reflected over the edge of the shell upon 
the free margin of the mantle, with which it is said to be organically 
connected. The middle or prismatic layer, according to Macalister, 
is absent in the shell of ^ijhwrium, 
it is however present in the genus 
Anodonta, forming about half the 
thickness of its valves, and is com- 
posed of numerous polygonal prisms 
set oblicpiely to the surface of the shell 
and forming a very densely calcified 
layer; the nacreous layer is usually 
about the same thickness as the 
prismatic layer, but it is in this re- 
spect to a certain extent dependent upon and proportionate to the 
age of the animal, it lines the whole internal surface of the shell. 
Fig. 92. — Oblique section through 
the shell of Unio (magnified), showing 
the three layers of which the shell is 
composed and the comparatively enor- 
mous development of the nacreous layer 
(after Semper). 
Cy periostracum or epidermis, the ex- 
ternal layer ; p. prismatic or middle 
layer ; «. nacreous or inner layer. 
