418 MARINE INVERTEBRATES 
and anterior margins, the shells are said to be equilateral. When 
the two valves are almost exactly alike in size and shape they are 
called equivalve. 
In many forms there is a heart-shaped space upon the dorsal 
margin of the closed valves forward of the umbones, called the 
Iwutle. Similarly placed back of the umbones is sometimes a 
more or less clearly defined space called the escutcheon. The outer 
surface of bivalve shells admits of infinite variety of sculpturing 
and ornamentation, ranging from a perfectly smooth to a heavily 
ribbed, nodose, spinous, or deeply decussated surface. The color- 
markings are often brilliant and eccentric. 
The right and left valves may be distinguished by remember- 
ing that the siphons are always posterior, and that the umbones 
usually point forward. 
The hinge of bivalve shells undergoes many variations in the 
different genera. The simplest type is a smooth edentate surface 
where the two valves meet and are held together only by the 
strong ligament. Generally, however, there is a system of inter- 
locking teeth, those in the center of the hinge being the cardinals 
and those upon either side the laterals. In some forms there are 
rows of fine comb-like teeth along the hinge-margin, with no dis- 
tinction between cardinals and laterals. 
The object of these hinge-teeth is at once obvious, for they 
give a rigidity and increased strength to the entire mechanism, 
especially when closed. There is occasionally a saucer-shaped 
plate or platform just under the cardinal teeth and within the 
shell, called the fossette. Upon this rests the internal elastic 
cartilage. 
Upon the smooth interior surfaces of the valves are always cer- 
tain markings, which correspond to anatomical features of the 
animal and are consequently of considerable importance in the 
determination of the systematic position of the specimen. First, 
the well-marked impressions left by the adductor muscles will be 
noted; then a more or less distinct line which, describing roughly 
a diucctn, connects the two muscle scars. This line represents the 
points at which the mantle became detached from the shell. In 
other words, all the space within this pallial line (barring the 
