42 AXEL A. OLSSON 



latter name is used if the resilifer has the shape of a spoon or a projecting 

 armlike process. The ligament is now considered "internal" since it lies con- 

 cealed within the hinge plate. In the case of the Mactridae, the resilifer 

 has the shape of a large, pear-shaped depression or cup in the middle of 

 the hinge plate, between the cardinal teeth, and below the main fulcrum of 

 the hinge. This internal position places the resilium under compressional 

 strain while the valves are held shut by the pull of the adductor muscles. 

 In structure, the resilium is fibrous or lamellar, often showing a pearly or 

 silky luster along a fractured surface; it is always impregnated with lime 

 giving to it a grayish color and resistance towards alkaline solutions; in 

 some families of the Anomalodesmacea, particularly, the Lyonsiidae and 

 Periplomidae, the middle portion of the resilium is often replaced so com- 

 pletely with lime as to form a shelly plate (the lithodesma), its purpose 

 is to strengthen the whole. The outer band or tensilium is usually much 

 thinner, often a mere skinlike membrane, brown in color, and of a chiti- 

 nous or hornlike nature. Like the periostracum, of which it is a continuation, 

 it is acted upon by strong, alkaline solutions. The mechanical action of 

 the outer layer of the ligament is mainly tensional or by its contraction 

 pulls the valves open by a shortening of the space above the fulcrum of 

 the hinge. The name "tensilimn" is here proposed for this outer part of the 

 ligament, to complement or conform in principle of naming with the 

 resilium (Dall), whose mechanics of operation is resistance to compression. 

 Hence, the ligament is under both tensional and compressional strain when 

 the valves are tightly closed. 



In most pelecypods, the dorsal margin of the valves bears small rough 

 projections or teeth which fit into sockets and form a swinging or hinge 

 union. These structures are called hinge teeth, and their shape and distri- 

 bution forms a set pattern characteristic for each family and genus. Illus- 

 trations of various hinge patterns will be found amongst the plates of this 

 work. The teeth set under the beaks and which appear to radiate out from 

 there are called cardinal teeth; those more removed or placed along the sides 

 are known as lateral teeth. In some families, the teeth may degenerate, be- 

 come partially or wholly obsolete as in some genera of the Lucinidae. A taxo- 

 dont hinge is one composed of many small teeth arranged in a more or less 

 unbroken series (Nuculidae, Arcidae); this type of hinge has sometimes been 

 considered as the more primitive. A cyclodont hinge is one without a definite 

 plate, the teeth arising as hooks or prongs from the margin; this type of 

 hinge generally shows evidence of torsion (Cardiidae). The most advanced 

 type of hinge is found in the Mactridae and Veneridae and is knowrrf as 

 teleodont; this type has well-developed cardinal and lateral teeth seated 

 on a definite platform-like plate. Some groups have an edentulous hinge 

 or one without any teeth; this type is characteristic of some borers. 



On the inside of the valve, there are several impressions which repre- 

 sent the attachment scars of various muscles. Most pelecypod shells have 

 two adductor muscle scars (Dimyarian) located at the anterior and posterior 

 ends of each valve. The two adductor scars may be of nearly equal size, 



