234 REPORT ON ALBATROSS MOLLUSCA DALL. 



of tbe organic characters must be our guide in attempting to decide. 

 Only too often we may find, as knowledge increases, that our first judg- 

 ment was more or less in error. 



In reflecting upon the origin of the complicated mechanical arrange- 

 ments in bivalves which we call the hinge, I have come to the conclu- 

 sion that here, as in the cases of the mammalian foot and tooth, elabo- 

 rated so clearly by Cope and Ryder, we have the result of influences of 

 a mechanical nature operating upon an organ or apparatus in the pro- 

 cess of development. 



The hinge of a bivalve, reduced to its ultimate terms, consists of two 

 more or less rigid edges of shell united by a flexible membrane or 

 ligament. 



The ligament may be wholly external or may be supplemented 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 connecting 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 edentu- 

 lous margin closing by simple apposition of the edges of the two valves; 

 (2) the hinge in which the teeth are developed in a direction trans- 

 verse to the cardinal margin; and (3) the hinge in which the direc- 

 tion 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 not difficult to determine on 

 a general survey of all its organic characters. 



lam disposed to think that the time relations of the different 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 architypal form of bivalve may be imagined as small, with nearly 

 equilateral, symmetrical, subcircular 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. 



Very few of the earliest known bivalves appear to have hinge teeth, 

 though this may be on account of our imperfect knowledge of many of 

 them, since they are often represented by fossils in which no evidence 



