INSTITUTE OF SCIENCE, PHILADELPHIA. 499 



corresponding to its range of expansion. Beyond the normal range in each 

 case the ligament and resilium operate in the reverse sense of that which is 

 respectively their ordinary function. 



The ligament proper (which will be called simply the ligament, in this 

 paper) is of a more horny nature, tough, and semi-translucent beneath 

 its external surface. When dry it has a vitreous fracture and often shows 

 hardly any fibrous texture. Its tendency when functional is to break in 

 planes at right angles to the axis of motion rather than in any other direc- 

 tion. 



The resilium is distinctly lamellar or composed of parallel fibres, which 

 are apt to give a pearly sheen to its broken surface. There is often a more or 

 less extensive intermixture of lime in its substance, which may be diffused, or 

 may be especially concentrated along the median plane, which is in many cases 

 also the smallest or most constricted part of the cartilage. As may be seen by 

 looking at the unbroken resilium, in Mactra, this organ in such cases has 

 something of an hour-glass shape, the ends which fit into the " cartilage " pits 

 or resilifers being more expanded than the centre between them. The deposit 

 of lime in the form of an accessor}^ shelly piece, usually termed the ossiculum 

 or /?V/i(?(/,£'j'7«i2;, has for its purpose the reinforcement of the resilium. In those 

 cases where the resilium and ligament have not separated, as, for instance, in 

 Unio, it will often be found that a considerable amount of granular limy mat- 

 ter has been deposited on and among the fibres corresponding to the resilium 

 below the ligament. When fresh this may easily be detached, leaving the 

 tougher ligament above uninjured. 



In the Paleopelecypod the ligament was short and central between and 

 below the beaks of the valves. With the lengthening of the hinge line it be- 

 came elongated ; with the development of asymmetry in the hinge it was shifted 

 forward or back from the beaks, according to the particular case. The long 

 hinge-line is more effective than a short one ; the long line, for reasons men- 

 tioned elsewhere, is more likely to be developed behind the beaks than in front 

 of them. Consequently the influence of natural selection may be surmised in 

 the process of development, which has resulted in the almost universal pres- 

 ence of the ligament on the posterior dorsal line of the shell, or at least behind 

 the umbones. The greater efficiency of a long ligament would tend, when 

 once it had been developed, to maintain it and with it the greater length of 

 the side of the hinge where it was seated. However, the rule is not universal, 

 and the nearer we approach the origin of the Pelecypoda the more numerous 

 the exceptions appear, while among those recent groups, which include the 

 more archaic types, several important divergencies may be noted. 



For the type of ligament which extends on either side of the beaks, Neu- 

 mayr adopts the designation amphidetic , while for the more perfected type 

 which has been withdrawn wholly behind the beaks he uses the term opistho- 

 detic. Among modern forms Pectuncidus offers a conspicuous type of the 



