490 TRANSACTIONS OF THE WAGNER FREE 



of the adductors developed, would probably present outlines more elongate or 

 triangular. The differences of form would necessarily react upon the develop- 

 ing hinge, from the inevitable action of physical laws, and thus tend to pro- 

 duce in connection with particular lines of evolution of form, particular types 

 of hinge. I have thought that the development of such diverging types of 

 hinge, the pedigree of existing types, would be useful in the classification of 

 the Pelecypoda, and provisionally applied these principles in a tentative classi- 

 fication proposed early in 1889 (Bull. U. S. Nat. Mus. No. 37, p. 26, May, 1889; 

 and Am. Journ. Sci., Dec. 1889, p. 460). That the tracing out of the line of 

 descent of several general types of hinge (without reference to the specializing 

 effect of later conditions, which often partially mask the type, as in other lines 

 of development) was a sound method, was shown effectively by the fact that the 

 classification derived from it harmonizes in essentials with that which is the 

 result of the most modern researches on entirely different lines; subtraction 

 being made of whimsicalities due to narrowness of view or want of breadth in 

 morphological knowledge. As the multitude of fossils can only be known to 

 us by their hard parts preserved in the rocks, a classification which enables us 

 to follow the line of descent approximately by characters still preserved by the 

 fossils, is not without special advantages to the biologist. If also this classifi- 

 cation be morphologically acceptable on other grounds, there would seem to 

 be no good reason why it should not be generally adopted, with the under- 

 standing that the first attempts at framing it must be subject to correction as 

 our knowledge of the facts increases. 



The Hinge Teeth. — Two functions are performed by the hinge teeth, 

 each of which has its own dynamic correlations. The first is the guiding back 

 of the closing valves, so that the first effort will bring the appropriate parts into 

 exact apposition. This function is fulfilled especially by long, more or less 

 curved teeth, like those of Yoldia or Cienodonta among the Prionodonts, and 

 Cardium among the Teleodonts. 



The second function is the preservation of the apposition after the valves 

 are closed, or, the prevention of any rotatory motion of either valve upon the 

 other in the marginal plane with the ligament as an axis. This function is 

 particularly important in shells which have a smooth inner basal margin and 

 full, equal valves, since such shells are more liable to unequal pressure affect- 

 ing only one of the valves or one valve more than another. Teeth may be 

 quite inefficient to bring about this result, which may, nevertheless, serve very 

 well as guides. For instance the long cardinal teeth of Cardium make excel- 

 lent guides, but help little to prevent rotation. This defect in Cardium is made 

 up for by the presence of lateral teeth, and more especially in many of the spe- 

 cies, by a denticulated basal margin.* 



*AmoDg all the numerous species of Cardium, only one or two are found with teeth obsolete and smooth 

 inner basal margins, and these are chiefly restricted to a sub-Arctic habitat, where enemies are perhaps fewer 

 than in warmer seas, and where the form seems to have persisted chiefly through its great fertility in indi- 

 viduals, 



