478 THE PALEONTOLOGY OP MINNESOTA.. 



LTermiology. 



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 j^oints 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. 



Lunule: A similar, but shorter and commonly heart-shappd 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 Amhonychiidce 

 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 Tellinomya, 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. 



