CLASS I PELECY PODA 349 
this reason is called the cardinal margin. It comprises the teeth, or project- 
ing processes and sockets, usually alternating in the single valve, and opposite 
with respect to both valves. In the more modern and perfected types, the 
cardinal margin is reinforced, by a vertical deposit of shell in the form of a 
lamina called the hinge plate, upon which the teeth are set. Above the hinge 
plate in each valve rise the beaks or umbones, which are usually curved 
toward the anterior end of the shell (prosogyrate), but are sometimes directed 
backward (opisthogyrate) or outward (spirogyrate). 
According to the ordinary terminology, the height of a Pelecypod is 
measured on a vertical from the beaks to the ventral margin; the length 
corresponds to the greatest distance between the margins parallel with the 
antero-posterior axis above defined ; and the thickness, or diameter, is measured 
by a line at right angles to the vertical plane descending from the cardinal 
axis (Fig. 592). When the shell is placed with the oral end anterior, the 
valves are termed right and left respectively, as viewed from above the 
articulus. The portion of the shell anterior to the beaks is usually shorter 
than that behind them, except in such forms as Donaz or Nucula. 
Viewed laterally, most Pelecypod valves may be divided into regions, 
corresponding in the main to the disposition of the internal organs. The oral 
area extends from the anterior end of the cardinal line to the anterior side of 
the pedal area. The latter is often marked by a swelling of the valves, and 
sometimes by a sinus (Pholas); it extends backward to a point where the 
branchial crest, radiating from the beaks, forms the anterior boundary of the 
siphonal area. The dorsal or posterior limit of the siphonal area is marked by 
an angle in the incremental lines ; and above this, extending to the posterior 
end of the cardinal line, is the intestinal area. In the alate forms, like Previa, 
the wings usually called anterior and posterior are really, with reference to 
the antero-posterior axis of the animal, dorsal and ventral. 
In certain borers, the siphons are greatly produced outside the valves, and 
a calcareous tube is secreted, lining the burrow; the valves; situated at the 
anterior end of the boring, either lie free, or are partially or wholly fused 
with the tube. In the Pholadidae the naked portions of the animal between 
the edges of the valves are often protected by additional shelly pieces, which 
are organically separate from the valves; and some burrowers have the free 
ends of the siphons protected by leathery or calcareous shields. In the 
Teredinidae these shields are specially modified to protect the entrance of the 
burrow, and are called “ pallets.” 
Ornamentation.—The external ornamentation of the valves is always a 
conspicuous character. It comprises (beside the concentric or incremental 
lines which indicate the successive additions to the shell margin, and are 
believed to coincide with resting stages during the process of growth) radial 
or concentric striae, ridges, ribs, folds, nodes, spines, or foliaceous processes. 
These are supposed to arise from temporary or permanent modifications of 
the mantle margins, such as papillae, minute tentacular, or proliferate pro- 
cesses. Other modes of ornamentation, such as that observed on the beaks 
of Unionidae, proceed from the serrate margin of the glochidial shell. 
Above the hinge line, in archaic types, is an area often set off by an 
impressed line and called the cardinal area. In the more perfected modern 
forms this area is commonly divided; a heart-shaped space in front of the 
beaks, and bounded by a ridge or groove, being known as the lunule ; and a 
