Dull — Hinge of Pelecypods and its Development. 44 7 



ties among which the sum total of the organic characters must 

 be our guide in attempting to decide. Only too often we may 

 find, as knowledge increases, that our first judgment was more 

 or less in error. 



In reflecting upon the origin of the complicated mechanical 

 arrangements in bivalves which we call the hinge, I have come 

 to the conclusion that here, as in the cases of the mammalian 

 foot and tooth, elaborated so clearly by Cope and Ryder, we 

 have the result of influences of a mechanical nature operating 

 upon an organ or apparatus in the process of development. 



The hinge of a bivalve, reduced to its ultimate terms, con- 

 sists of two more or less rigid edges of shell united by a flexi- 

 ble membrane or ligament. 



The ligament may be wholly external or may be supple- 

 mented by an internal addendum (called the cartilage), which 

 exerts a stress in the same direction within certain limits. The 

 movements of the hinge are dependent upon the elasticity of 

 the ligament and cartilage and upon force exerted by one or 

 more adductor muscles uniting to the valves 



The rigid edges or cardinal margins of the valves may be 

 simple or modified by the presence of interlocking processes 

 known as teeth, whose purpose is to regulate the direction of 

 the valves in opening and closing. 



There are three fundamental types of hinge : 1, the simple 

 edentulous margin closing by simple apposition of the edges of 

 the two valves ; 2, the hinge in which the teeth are developed 

 in a direction transverse to the cardinal margin ; 3, the hinge 

 in which the direction of the teeth is parallel to the margin. 

 The mechanical features of the second and third types may be 

 more or less combined in a single hinge, but the affinities of 

 the particular form in which this may occur are usually n3t 

 difficult to determine on a general survey of all its organic 

 characters. 



I am disposed to think that the time relations of the differ- 

 ent types are those of the order in which I have cited them ; 

 the most perfect hinge, morphologically speaking, would be 

 one which should combine the most effective features of the 

 second and third types. 



The archetypal form of bivalve may be imagined as small, 

 with nearly equilateral, symmetrical, sub-circular valves with 

 edentulous cardinal margin and a short external ligament 

 nearly central between the umbones. This is the character of 

 many larval bivalves at the present day, though it is probable 

 that many of the forms now edentulous in the adult state, have 

 passed through an evolutionary stage in which they had a more 

 or less denticulate hinge-margin, while their present condition 

 is one in which the hinge has diminished in complexity or, in 

 other words, undergone degeneration. 



