METAZOA MOLLUSCA. 205 



In passing through these stages the shell illustrates 

 Minot's law of growth ; /'. ^., growth decreases as the size 

 of the animal increases. In the younger stages growth 

 was rapid, the hinge extended in length and the shell 

 doubled its size in a brief time, but in the gerontic stage, 

 growth is limited and the hinge area is narrow, as seen 

 by tracing the edges of the layers from the broad, rounded, 

 posterior part to the comparatively short hinge line. 



Anomia (PI. 391, A. simplex, No. 392), is related to 

 Pecten though it is a much more specialized form. When 

 young it is found free and crawls by means of its large 

 foot which is seen extended in PI. 391, fig. 2. The very 

 young shell (fig. i, left or upper valve) has the prodisso- 

 conch on the edge. Fig. 2 is a somewhat older stage 

 seen from the left or upper side. The later dissoconch 

 layers of shell are beginning to encircle the prodissoconch. 

 Fig. 3 is the same shell as Fig. 2 viewed from the right 

 or lower side. The byssal notch is indicated on the edge 

 of the prodissoconch in which particular, Anomia differs 

 from Pecten, the latter having an entire prodissoconch. 

 The byssal notch was originally on the edge, but has 

 extended nearly to the center and is partly encircled by 

 the layers. The prodissoconch becomes entirely enclosed 

 by layers. Fig. 4 is an older stage which shows how the 

 encircling layers have pushed the prodissoconch inward 

 some distance from its original position on the margin. 



The byssal notch of the lower valve is finally com- 

 pletely enclosed by shell layers, as seen in fig. 5 (adult 

 Anomia, lower side ; fig. 6, side view of the same). 



At first, when the byssus is surrounded, the opening 

 or foramen is small, but it becomes larger by the resorp- 

 tion of the shell. 



Specimens of the adult are seen in No. 392 ; (a) is 

 the whole shell; (b) the upper valve showing the scaly 

 appearance due to the irregular layers of shell ; (c) is the 

 lower valve. 



The same trunk forms, Nucula and Rhombopteria, 



