242 REPORT ON ALBATROSS MOLLUSCA DALL. 



hinge at one end (as in Macrodon), so that part of a row of teeth origi- 

 nally similar would come to differ from the rest. Many NucuJacea 

 show stages of such a mode of change. 



Another type would arise from the plications of the hinge parallel to 

 and induced by the formation of a fossette or process for the internal 

 cartilage. Such teeth or plications may be observed in most Pelecy- 

 pods haviug an internal cartilage. All stages in development of this 

 type may be observed from the barely traceable parallel ridges of Gus- 

 pidaria, for instance, to the highly specialized cardinal teeth of Mactra. 



Thus, it wiljl be observed, the teeth called cardinals in Pelecypods 

 are by no means all necessarily homologous, and it is even conceivable 

 that cardinals of both types might come to be united in the same 

 hinge. 



The development of lateral teeth from transverse teeth is a very easy 

 process, of which a full exhibit might be made by arranging in a con- 

 tinuous series the valves of selected Arcacea and Nuculacea. It is prob- 

 able, however, that not all Orthodont dentition originated in this way. 

 The thickening of the cardinal margin rendered necessary by the stresses 

 involved in the mechanical operation of cardinal teeth or strong exter- 

 nal ligaments would render parallel plication of the thickened area 

 along the margin not only easy, but almost inevitable in some cases. 

 The infolding of the edge of the mantle necessarily accompanying the 

 production of a strong specialized socket for an internal cartilage 

 would lead incidentally to occasional deposition of shelly matter in 

 ridges parallel with the longer edges of such sockets. The greater 

 efficiency in guiding the valves to effective closure in proportion to 

 the increased distance from the umbonal region of such interlocking 

 plications would tend through natural selection to the perpetuation of 

 favorable variations and to their gradual removal farther and farther 

 from the beaks until the most useful distance was attained. 



When we consider the remarkable uniformity in hinge characters 

 attained by the species with more perfected forms of hinge, through 

 long series of individuals, it seems almost incredible that these results 

 should be brought about by the action of a thin, soft film of secretive tis- 

 sue which, unaided, could not hold itself erect. It is only when we re- 

 member that the result, in the main, is brought about through the action 

 and reaction of certain defiuite mechanical stresses, propagated through 

 the hard valvular skeleton, and constantly imposed upon the softer tis- 

 sues, that any adequate reason for the marvelous uniformity presents 

 itself. There are certain groups, such as the Isocardiidw, in which the 

 hinge seems still to be iu what may be termed a transition state. With 

 these no such strict uniformity prevails. While the differences are not 

 excessive, yet the hinge of each individual specimen compared with 

 others of the same age will show individual characteristics, and the 

 changes which the hinge undergoes in the same individual between 

 adolescence and old age are greater than one would ordinarily find in 



