478 THE PALEONTOLOGY OF MINNESOTA. 
{Termiology. 
margins. This line may be parallel with the hinge, but more commonly 
diverges more or less strongly posteriorly. The hight is given in one or two 
measurements; the former, when the shell is approximately equilateral (as in 
many species of Tellinomya) or elliptical in form (Clidophorus) with the greatest 
hight subcentral or beneath the beaks; the latter, when the shell is elongate 
or has one end much wider than the other (Orthodesma and Modiolopsis). “In 
such cases two lines are drawn at right angles with the hinge line, one from 
the beaks to the ventral margin, the interval between the two points being 
the anterior hight, the other across the posterior end, from the posterior 
extremity of the hinge, giving the posterior hight. By thickness is understood 
the shortest distance between the points of greatest convexity of the valves. 
Area or Escutcheon: A variously shaped, usually elongate, inflection of the dorsal 
edge, generally longitudinally lineate, and serving as a receptacle for the 
ligament. When the area is restricted to posterior to the beaks, as in 
_ Cuneamya, it is, strictly speaking, to be called an escutcheon. 
Inunule: A similar, but shorter and commonly heart-shaped inflection or distin- 
guishable area in front of or beneath the beaks. Cuneamya offers good 
examples, 
Gaping and closed shells: The valves fit either closely all around or they may fail 
to do so and gape at one or both ends, and sometimes ventrally. 
Byssal opening: A small, distinctly modified portion of the. anterior margin, through 
which the byssus protruded. Among the paleozoic types the Ambonychiide 
furnish the best examples. 
Beak: A more or less prominent point on each valve, usually bending forward and 
overhanging, in a variable degree, the dorsal edge. It marks the point at 
which growth began, and generally is situated anterior to the center of the 
valves. Many species of Te/linomya, Nucula and Clinopistha afford exceptions 
to the last rule. 
Umbones: The use of this term, which is generally applied in a sense synonymous 
with beaks, is here restricted to the gibbous rostral portion of valves in which - 
the beaks are incurved over the hinge line and invisible in a side view. 
Umbonal ridge: A more or less strongly rounded or angular ridge-like prominence, 
extending from the beaks or umbones toward the posterior extremity of the 
shell. Example, Whitella. 
Cardinal or dorsal slope: Generally applies to the flattened or concave declivity 
from the umbonal ridge to the dorsal edge posterior to the beaks, When 
the declivity on the anterior side is sufficient to be noted it is designated. as 
the anterior cardinal slope. 
