486 TRANSACTIONS OF THE WAGNER FREE 



dynamics of its functions, and the directions and causes of its development and 

 present modification. 



The Valves. The bivalve shell reduced to its lowest terms comprises 

 two convex pieces attached to one another dorsally (i) by an elastic ligament 

 external to the cavity between the valves; and (2) by muscles and connective 

 tissues which p.ivs ' from the inner surface of one valve to the inner sur- 

 face of the'op'posite valve. By the contraction of the muscles the margins of 

 'he'v.ilves ;>re brought into close contact, thereby forming a hollow receptacle 

 in which the soft parts of the animal may be enclosed and obnoxious matters, 

 living enemies, or inorganic material liable to irritate the tissues, may be com- 

 pletely excluded. The elasticity of the ligament, acting on the principle of a 

 C-spring, tends to separate the valves when the tension of the internal adduc- 

 tor muscles is relaxed. The substance of the valve is secreted by marginal 

 glands around the edge of the investing tissue or mantle, and is subsequently 

 reinforced by material supplied by secretion from the general surface of the 

 mantle ; each secretion having a special character, distinguishable in the struct- 

 ure of the shelly deposit. As the animal grows and the original prodissoconch 

 becomes too small to cover the soft parts the valves are enlarged, always at 

 the margin, so that each valve, fundamentally, represents a hollow cone. As 

 growth progresses more rapidly along some portions of the mantle than at 

 others the cones necessarily become oblique, arched, or cycloidally curved. 

 The apex of the cone is the beak or umbo of the shell, the base is the entire 

 margin of the valve. Owing to the necessarily asymmetrical disposition and 

 mode of growth of the various organs of the body, the subcircular outlines of 

 the prodissoconch cannot long be maintained, and the valve becomes modified 

 in outline according to the circumstances of each particular case. 



The original protopelecypod was small, thin, symmetrical, subcircular or 

 oval, with a short external ligament * equally disposed on each side of the 

 beak along a line (the hinge-line) constituting the axis of motion for the valves 

 in opening and closing. The mantle was not uniformly attached to the shell 

 along a pallial line, as in modern pelecypods, but adhered more or less irregu- 

 larly and was not provided with extrusile siphonal tubes. The adductor 

 muscles were subequal, symmetrical, and situated high up in the valves. The 

 surface of the valves was smooth, or (probably in connection with the develop- 

 ment of tactile papillae on the mantle edge) radiately ribbed. These conclusions 

 are justified not only by theory and by the recent investigations of Jackson 

 and others on the morphology of the prodissoconch, but by the characters of 

 the oldest known pelecypoda, summarized by Neumayr under the name of 

 Paleoconcha. 



In a limited group, generally characterized by the incubation of the young 

 fry within the valves, the anal chamber, or even in a specially developed bran- 



* Throughout this work "ligament" refers to the external, and " resilium " to the relatively or actually 

 internal bond between the dorsal margins of the valves. 



