30 Marine Shells of the Western Coast of Florida 



margin; the height or altitude is measured by a line from the umbos 

 to a point immediately opposite on the ventral margin. Thickness 

 or diameter is the greatest dimension between the two sides of the 

 closed valves. 



In most pelecypod shells the two valves are held together be- 

 low the beaks by a hinge consisting of interlocking teeth — depres- 

 sions in the margin of one valve receivmg the teeth from the oppo- 

 site valv^e. These teeth are distinguished as cardmals, which are those 

 immediately below the umbos; and the anterior and posterior laterals, 

 which are on the respective sides of the cardinals. The teeth are not 

 always identical in every species of a particular genus, but show 

 various modifications in development and position. The hinge liga- 

 ment is a tough band which fastens the two valves together along 

 a line immediately adjacent to the umbos. This ligament is com- 

 posed of two distinct parts; the outer portion — the ligament proper 

 — which is usually external and may be seen when the valves are 

 closed; and the inner portion which is cartilaginous and elastic. The 

 ligament proper is inelastic, while the cartilage is highly elastic and 

 compressible, and in its position of rest tends to keep the two valves 

 of the shell slightly separated. Together the two parts of the hinge 

 act in opposition to the strong adductor muscles which close the shell 

 and tend to maintain approximation of the valves so long as they 

 remain contracted. 



The more or less heart-shaped depression, close to and ante- 

 rior to the umbos is the lunule. A corresponding area posterior to 

 the umbos is the escutcheon. The latter occurs in but few genera. 



The inner surfaces of the valves show certain impressions or 

 scars, marking the sites of attachments of adductor muscles. When 

 there are two muscles there are two impressions in each valve, the 

 anterior and posterior muscle impressions. When there is but one 

 adductor muscle scar, it is the posterior which is present. The line 

 which roughly parallels the shell margin some little distance from 

 the edge is the pallial line, and the inward recession or indentation of 

 this line is the pallial sinus. 



To distinguish between right and left valves, the shell is held 

 with the ventral margin downward and the umbos pointing away 



