456 Dal! — Hinge of Pelecypods and its Develoj)?nent. 



to the highly developed and specialized cardinal teeth in 

 Mactra. Thus it will 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 hinge of a single species. 



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 continuous series the valves of selected Arcacea 

 and NuGidacea. It is probable however, that not all Orthodont 

 dentition originated in this way. The thickening of the car- 

 dinal margin rendered necessary by the stresses involved in the 

 mechanical operation of cardinal teeth or strong external liga- 

 ments, 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 dis- 

 tance 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 char- 

 acters attained by the species with more perfected forms of 

 hinge, through long series of individuals, it seems almost in- 

 credible that these results should be brought about by the 

 action of a thin soft film of secretive tissue, which, unaided, 

 could not hold itself erect. It is only when we remember that 

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

 and reaction of certain definite mechanical stresses, propagated 

 through the hard valvular skeleton' and constantly imposed 

 upon the softer tissues, that any adequate reason for the mar- 

 vellous uniformity presents itself. There are certain groups 

 such as the Isocardiidce in which the hinge seems still to be in 

 what may be termed a transition state. With these no such 

 strict uniformity prevails. While the differences are not ex- 

 cessive, yet the hinge of each individual specimen compared 

 with others of the same age will show individual characteris- 

 tics and the changes which the hinge undergoes in the same 

 individual between adolescence and old age are greater than 

 one would ordinarily find in the whole membership of a 

 species, say of the Veneridw, taking all ages, above the larvaL 

 stage into account. 



