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THE AMERICAN NATURALIST [Vol. XL 



loss in the power of growth, beginning almost immediately after 

 birth." This same decrease is very realistically shown in the 

 little gastropod, Litorina littorea, so abundant on our x\tlantic 

 coast. This shell, in the vicinity of Boston, is very quickly at- 

 tacked by an alga which discolors and erodes it. So if a series 

 of the shells from small to large is collected at mid- or late sum- 

 mer before the new growth has become corroded by the algse, 

 the amount of that year's growth is very distinctly shown. Such 

 a series shows that, while on the small specimens the year's growth 

 was more than two complete whorls, in older specimens it became 

 progressively less until in some of the mature ones it was but 3.5 

 mm. Finally on the older shells growth was extremely reduced, 

 being on one shell only 0.75 mm. For these facts concerning 

 Litorina we are indebted to Professor R. T. Jackson whose series 

 of these shells collected from Manchester, Mass., shows the above 

 facts. The series is now on exhibition at the Boston Society of 

 Natural History. 



This relative decrease in growth is also shown in the crowding 

 of the septa in old-age cephalopods. Among pelecypods and 

 brachiopods the relative decrease in the amount added to the shell 

 is indicated in the more crowded condition of the later added 

 growth lines. For example, a specimen of Atrypa reticularis, No. 

 641, shows 12 growth lines on the gerontic portion which give 

 a thickness of 5 mm. while the preceding growth, about 22 mm. 

 long, has also only 12. Yet if the growth lines were added at 

 regular time intervals the gerontic stage represented as long a 

 period as that from embryonic through ephebic. 



That the more prominent growth lines may define the shell 

 growth for definite periods of time is indicated in the following 

 examples. Buxbaum showed that Anodonta cellensis, one of the 

 UnionidfE, had two strongly marked concentric lines and hence 

 three sets of more faintly marked areas, and this shell was known 

 to be three years old (Latter :04, p. 163). 



The common oyster commercially marketed is about four years 

 old when gathered. Blue Points, which are smaller, are three 

 years old. This age is broadly indicated on the shells by the 

 stronger growth lines. On the Litorina cited above, the new 

 growth is usually bounded posteriorly by a prominent growth 

 line. 



