Dall — Hinge of Pelecypods and its Development. 449 



or angulated. For obvious reasons a ligament forming a curve 

 or the arc of a circle is mechanically impossible This any- 

 one may prove to his own satisfaction by putting two light 

 wooden saucers edge to edge, convexity outward, and attach- 

 ing a leather or paper ligament by cement. A curved liga- 

 ment when the "valves open will tear or break at once, either 

 itself or the edge to which it is fixed. In other words, "the 

 axis of motion of the hinge must be in a straight line. If any 

 part of the ligament diverges from the axial line, it must cease 

 to take part in the axial motion and must be capable of stretch- 

 ing to an extent which will neutralize its angulation, or it will 

 be broken or torn away. But if the thickness of the ligament 

 increases ventrally, as may be the case when it is situated 

 between the valves rather than as an arch above them, a 

 certain portion may extend to and beyond the axial plane in a 

 downward direction. The portion thus' projecting will then 

 partake of the axial motion in an opposite sense to that portion 

 which remains above the axial line. It will be compressed 

 when the latter is stretched by the closing of the valves and 

 will expand as the opening of the valves allows the external 

 portion to contract. This change may be brought about by a 

 downward angulation of one end of the ligament (as in Soleno- 

 rnyd) or as a simple downward growth, which may be central 

 (as in Neilonella or Galeomma). The former may be the 

 result of an angulation of the hinge-margin consequent on 

 elongation or ventral extension. Its result is to separate a 

 terminal segment of the original ligament, which segment may 

 be totally detached or remain physically connected; while in 

 either case its mechanical function has undergone a reversal of 

 direction. 



The second mode likewise removes a segment but in a 

 vertical direction. This segment may be physically continu- 

 ous throughout its upper portion with the lower portion of the 

 superjacent ligament, it may be wholly detached, or it may be 

 attached by one extremity while the other is separated ; in the 

 last case its direction will be oblique or at an acute angle with 

 that of the original ligament. This detached segment what- 

 ever its position has always similar mechanical relations to the 

 movement of the hinge and is called the cartilage. The sepa- 

 ration of the cartilage from the ligament is generally either 

 central or toward the shortest end of the hinge, which is usu- 

 ally the anterior, owing to the fact that when the size of a 

 lamellibranch increases, the siphons, the ovaries, the visceral 

 mass or the gills are the organs where proportionally increased 

 growth is most likely to occur, and these are usually central or 

 posterior to the umbones. In Solenomya, which is exceptional 

 in having a posterior cartilage, the posterior portion is shortest. 



