Jackson.] 544 [April 4, 



seen from above, in the initial stages of spat growth, is a notch 

 which indicates the origin of the ligamental groove of the lower 

 valve. This is seen clearly in fig. 9, pi. v. 



It is necessary to describe the left and right valves separately. 

 The upper right valve, fig. 10, pi. v, is directly continuous with the 

 prodissoconch valve and follows the curve of that stage though in 

 a lessening degree. It spreads out laterally on the hinge line in a 

 descending curve. The lower left valve, fig. 11, pi. v, starts as 

 does the upper, following the curve of the early valve. When this 

 curve has been followed for an extremely brief period, the valve 

 suddenly flattens, and becomes closely related to the surface of at- 

 tachment. The result of this, is that a slight groove runs around 

 the border of the spat valve, beyond which it is abruptly contin- 

 uous with the flat later growth, as has already been mentioned by 

 Professor Ryder (23). I have, however, to note a peculiar and 

 significant exception. Sometimes instead of becoming flat and 

 closely related to the object of attachment, the spat valve of this 

 stage is continuously curved throughout its extent, and not at all 

 flattened. It then closely resembles the upper valve of the same 

 stage in shape. This curved shape is held, as far as I have made 

 out, only on rough objects of attachment. Seeking for such mate- 

 rial on rough stones and shells, one will always find some valves 

 more or less curved in their earliest stages. This is very interest- 

 ing, as showing the modifying influences of environment, on a very 

 young stage where heredity might be supposed to fashion the form, 

 with overpowering force. 



Second silphologic stage (figs. 12, 13, pi. vi). — In the second stage 

 the upper valve is in form a continuation of the first stage. The 

 lower valve when flat begins to spread out over the object of attach- 

 ment by wing-like extensions of its anterior margins. In the lower 

 valve this stage commonly makes a shoulder, on its anterior border 

 by its increased growth, where it comes in contact with the outer 

 limit of the first silphologic stage, fig. 13, pi. vi. Similar shoulders 

 also mark the periods of growth of later stages in the lower valve. 

 The lower valve may remain concave throughout this stage as in 

 the first silphologic stage. The shells during the first two stages 

 are characteristically even and rounded in outline. 



Third silphologic stage (fig. 14, pi vi). — The upper valve in 

 smooth regular growing specimens still holds the simplicity of out- 

 line and conforms closely to the angles of curvature laid down in 



