ORGANS OF THE SPAT 59 



Dissoconch (spat shell) Prodissoconch (larval shell) 



58 X 50 53 X 47 



58 X 58 52 X 52 



63 X 62 55 X 54 



82 X 75 52 X 51 



83 X 70 57 X 53 

 90 X 79 55 X 54 



105 X 95 52 X 52 



110 xl05 52 X 52 



115 xll2 54 X 54 



The substance of the spat shell is doubtless deposited by the thickened 

 rim of the mantle in layers along the ventral and terminal edges of the 

 larval shell, but not to any conspicuous extent along the dorsal or hinge 

 edge, where what is deposited serves more to increase the thickness of the 

 shell and to press the umbos farther apart rather than to add to the height 

 of the shell. This explains why the prodissoconch remains near the 

 dorsal margin of the dissoconch during the growth of the latter, as well as 

 explains the concentric lines of growth below the umbos. The latest de- 

 posited parts around the margins are very thin and delicate and exhibit a 

 prismatic structure as if each prism were deposited by a cell. 



At first the shape varies little from that of the prodissoconch (Plate II, 

 figs. 1, 2), but soon the dissoconch becomes extended fore and aft of the 

 hinge area in a manner that suggests the wings or ears (alte) of a scallop 

 shell (Plate II, figs. 3, 4, 5), the lower parts preserving a pretty uniformly 

 curved outline. Later these alae cease to be conspicuous and the whole 

 outline may become irregular and variable. Deep or shallow concentric 

 creases preserve more or less indication of stages of growth, and at places 

 there may be portions of radial lines. The deeper concavity of the left 

 valve remains noticeable for a time after fixation takes place, particularly 

 in sections, but a little later the lower valve seems to lag behind the 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 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, is the anterior end. 

 The foot is postero-ventral to the velum. The umbos are postero-dorsal, 

 right and left. 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. After fixation it becomes difficult to retain 

 such ideals as continuously useful marks of description. At periods vary- 



