204 SYNOPTIC COLLECTION. 



young animal has reached the stage represented by fig. 6 

 the ends of the inner pair are turned inward while those 

 of the outer pair are reflected outward, as seen in fig. 7 

 which represents the gills of the adult. When young the 

 Pecten attaches itself by a byssus and always lies on the 

 right valve. After becoming attached it may detach itself 

 but soon becomes fastened again. When it reaches adult 

 life it is free, but keeps the same position with the right 

 valve below, and swims by clapping its valves together. 

 At this stage the hinge is toothless. No. 387 is a species 

 of Pecten showing the mantle, large muscle, and gills. 

 Pecten varius Linn. (No. 388), illustrates the unequal 

 development of the ears and the variation in color in one 

 species. No. "389 is a remarkably fine specimen of the 

 adult Pecten maximus,and PI. 390, figs, i, 2, are drawings 

 of the same. Here we have in one shell an epitome of a 

 great part of the life history of the group to which Pecten 

 belongs. 



The prodissoconch has disappeared, the peduncle hav- 

 ing usurped its place, but the tiny cavity (No. 389 ; PL 

 390, fig. 2) at the beak remains, telling of the rounded 

 outline and long hinge line of the embryonic shell. The 

 larval or nepionic stage is convex at first and smooth with 

 concentric markings. The hinge is long, while ears and 

 ribs are not developed. In the later nepionic stage the 

 beginnings of ribs are seen. At this time the shell is 

 light yellow in color. In the adolescent or neanic stage 

 the shell becomes concave and ribs are more developed. 

 The mature or ephebic stage is convex at first and the 

 ribs are prominent, while the color has changed to red- 

 dish. The later ephebic stage shows a tendency to return 

 to the concave condition which increases in the gerontic 

 stage. This is seen in the specimen and in fig. 2, but 

 still better in fig. i, which is a section through the middle 

 of the two valves, the lower valve being on the right. 

 The ribs tend to flatten out in the gerontic stage, while 

 the concentric markings become prominent and are nearer 

 together. 



