3r,o 



THE AMERICAN NATURALIST [Vol. XLIV 



upper one in growth, appearing thinner and flatter, while 

 the upper one is thicker and more curved. At a still 

 later period the growing edge of the lower valve becomes 

 free and the valve again acquires a deeper cavity than 

 the upper one, preserving this difference throughout life. 



While the developing oyster is free to swim or to creep 

 it is, of course, natural to describe it in terms suitable to 

 such permanently free-living species as the clam. The 

 more pointed end, that ordinarily precedes in locomotion 

 and from which may protrude the velum or the foot, is 

 the anterior end. The foot is postero-ventral to the 

 velum. The umbos are postero-dorsal. The hinge is 

 dorsal, i. e., between and in front of the umbos. The 

 longest diameter is horizontal, and the height is a vertical 

 line at right angles to the length. With the growth of 

 the spat it becomes difficult to retain such ideals as con- 

 tinuously useful marks of description. At periods vary- 

 ing somewhat with the individual they become more or 

 less modified. Preserving the original orientation of 

 the prodissoconch, the height of the dissoconch soon be- 

 comes greater than the length, the hinge and umbos come 

 to mark the narrow anterior end of the spat, and the 

 larval shell sinks into insignificance. Its left valve fre- 

 quently becomes obliterated by growth of the surface of 

 attachment, but its right valve may often be found until 

 late in the life of the spat, although it is liable to be- 

 come destroyed by weathering of the umbo-region. 

 While its position marks the anterior end of the oyster, 

 it has long since been carried up on the tip of the umbo 

 of the dissoconch out of reach of the hinge or of the 

 growing parts, but its anterior end still points in a gen- 

 eral way in' the direction of the anterior end of the adult. 

 This position and relationship is correlated, as will be 

 seen later, with the increase of growth of the lower and 

 posterior parts of the oyster's body, which occasions 

 more or less of a rotation round the great adductor mus- 

 cle, and resulting in longer or rounder forms of shell with 

 a straighter dorsal and a much curved ventral border. 



