714 
BULLETIN OF THE BUREAU OF FISHERIES 
known seasons of the year, first established the ring method of age determination. 
The accuracy of this method for trees has been demonstrated recently by the re- 
markable researches in the Southwest by Douglass (1929). 
Certain ridges, lines, or other marks appearing on or in the scales, otoliths, verte- 
brae, and other hard parts of fish have been determined by investigators to be of 
annual occurrence. Because of resemblances to the rings of growth of trees, these 
marks of growth in animals also have been called rings. 
Gilbert (1913), Frazer (1917), Rich (1920), Greaser (1926), and others have used 
the rings in scales in determining the rate of growth of fish. Crozier (1914, 1918) 
showed that the growth lines in the shells of chiton were significant in determining 
age. Weymouth (1923) established the association of the rings occurring in the 
Pismo clam with the retardation of growth during the winter period, and McMillin 
(1924) made use of the rings occurring in the razor clam in determining the normal 
course of growth in that animal. 
Hessing (1859), working with European species, was perhaps the first scientific 
writer to call attention to the possible correlation of certain rings in the shells of 
fresh-water mussels with the annual growth of these animals, although he was unable 
to decide definitely that these rings were of annual occurrence. Hazay (1881), fol- 
lowing the growth of individual Hungarian mussels, found that no growth occurred 
during the late winter months, resulting in the formation of a definite ring in the 
periostracum of the shells. As there was but a single major growth period each 
year, Hazay’s w r ork established these rings as annual marks. Lefevre and Curtis 
(1912), from their studies of specimens of the North American mussel, the pocket- 
book, Lampsilis ventricosa (Barnes), which they kept under observation for three 
years, conclude that the rings mark the boundaries of growth periods; but since vari- 
ous factors may cause cessation of growth, these writers were not entirely certain 
that a single growth period always corresponds to a single year. 
Rubbel (1913), after measuring over 300 specimens of a European fresh-water 
mussel, Margaritana margaritijera (Linnaeus), which he then planted, and remeasured 
two years later, also felt uncertain as to the significance of these rings as marks of 
annual growth. Isley (1914), from his study of some 900 specimens of North Ameri- 
can species of fresh-water mussels which he tagged and subsequently recovered, 
states that the winter rings, or arrested growth rings, as he recommends calling 
them, are usually sufficiently regular and definite to be used as indicators of age. 
He did not, however, make use of these rings in his own studies of the normal course 
of growth. 
Coker, Shira, Clark, and Howard (1921) held a number of fresh-water mussels 
at the United States Fisheries Biological Station at Fairport, Iowa, for periods of 
years and measured them annually. These authors made a very careful study of 
the growth rings by using sections of the shell. They pointed out that the growth 
of the shell in length and breadth is accomplished by the secretion by the mantle 
of the three layers of shell substance at or near the margin of the mantle. A period 
of cold or any disturbance, such as handling, causes the mantle to withdraw from the 
margin of the shell to such an extent as to break its continuity with the thin and 
flexible edge of the shell. When the deposition of shell is resumed, the new layers of 
prismatic substance and periostracum are not continuous with the old. The amount 
of overlapping of layers in the region of interrupted growdh appears to depend on the 
extent to which the mantle has been withdrawn, which in turn appears to depend on 
