236 REPORT ON ALBATROSS MOLLUSCA DALL. 



from the axial line it must cease to take part in the axial motion and 

 must be capable of stretching to an extent which will neutralize its an- 

 gulation, 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 be- 

 tween 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 down- 

 ward angulation of one end of the ligament (as in Solenomya) or as a sim- 

 ple downward growth, which may Ue central as in Neilonella or 

 Galeomma). The former mode may be the result of an angulation of the 

 hiuge margin consequent on elongation or ventral extension. Its re- 

 sult is to separate a longitudinal segment of the original ligament, 

 which inay be totally detached or remain physically connected, while 

 in either case its mechanical function has undergone a reversal of direc- 

 tion. 



The second mode likewise removes a segment, but in a vertical di- 

 rection. This segment may be physically continuous throughout its 

 upper portion with the lower portion of the superjacent ligament. It 

 may be wholly detached, or it inay 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, whatever its position, has always similar mechanical relations 

 to the movement of the hinge, and is called a cartilage. The separa- 

 tion of the cartilage from the ligament is generally either central or 

 toward the shortest end of the hinge, which is usually the anterior, 

 owing to the fact that when the size of a lamellibrauch 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 the 

 shortest. 



The amount of shifting required to put part of the ligament on the 

 ventral side of the axis of hiuge motion, or cardinal axis, is extremely 

 small. All stages of the changes involved may be observed in the Nu- 

 culacea, even to one, not hitherto mentioned, where the cartilage has 

 been developed and has subsequently become obsolete or altogether 

 disappeared (Malletia), while leaving some traces of its former presence 

 in the shape of an empty and degenerate fossette (Pleurodon). It is 

 noteworthy that this suborder, which gives us so many hints 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. 



