450 Dall — Hinge of Pelecypods and its Development. 



The amount of shifting required to put part of the ligament 

 on the ventral side of the axis of hinge motion, or cardinal 

 axis, is extremely small. All stages of the changes involved 

 may be observed in the Nuadacea, even to one not hitherto 

 mentioned where the cartilage has been developed and has 

 subsequently become obsolete or altogether disappeared (Mal- 

 letia), while leaving some traces of its former presence in the 

 shape of an empty and degenerate fossette {Pleurodori). It is 

 noteworthy that this suborder, in which the shell gives us so 

 many bints as to processes which we may imagine to be of 

 great antiquity, should on other grounds be regarded as among 

 the few which best retain traces in the soft parts of archaic 

 stages of development. 



With the lengthening and angulation of the cardinal margin 

 the ligament gradually shifted to a point where it became 

 posterior to the beaks. Perhaps it would be better to say that 

 the portion in front of the beaks either became segmented off 

 as a cartilage, or became obsolete and vanished, while the por- 

 tion on the posterior side gradually elongated, as the elonga- 

 tion of the posterior hinge-margin rendered a longer ligament 

 more useful. It has already been pointed out that a curved 

 ligament would involve stresses leading to its own destruction. 

 The curvature of the cardinal margin, now the common prop- 

 erty of a vast majority of bivalves, was inevitable with increase 

 in size and a symmetrical development of the anterior and pos- 

 terior ends of the body. Consequently that the ligament 

 should be shifted was a mechanical necessity, unless the evolu- 

 tion of the group was to be confined within extremely narrow 

 limits as regards hinge characters. 



The infolding of the ligament and the development of a car- 

 tilage and its supports would be especially likely to occur in 

 forms with a thin edentulous hinge, where the least shifting 

 would be necessary OSolenomya, Anatina) rather than in those 

 with a broad flat hinge-margin. In harmony with this propo- 

 sition we find the archaic forms with internal cartilage have 

 generally a narrow edentulous cardinal border, the exceptions 

 belonging to the more recently specialized types (Mactra, 

 Spondyhis) ; while the groups without an internal cartilage 

 contain the broadest and heaviest types of hinge (Pectunculus, 

 Vejieridw.) 



The infolding of a cartilage which arose by longitudinal 

 segmentation would leave a line of weakness in the arch of the 

 umbones. In thin shells with strong adductors there would be 

 a tendency to fracture here. This singular feature has been 

 perpetuated in what may be termed the normal umbonal 

 fissure of Solenomya, Periploma and similar forms. Traces 

 of it are evident in TAracia, while the unfractured suture it- 

 self is visible in Isocardia, Pachyris?na, Pecchiolia. 



