The criteria of an age mark as set forth previously (^Yalford and llosher, 

 I9U3) remains unchanged, namely; 



"An annulus is concentric with the narf^in of the scale. It 

 is not always a sharp or unbroken line; nor are the segments 

 of an interrupted annulus always perfectly co-circular (if 

 the shape of a scale may be called circular in this discus- 

 sion). But the course of an axinulus, continuous or broken 

 as it may be, can usually be traced, by careful scrutiny if 

 necessary, entirely around the sculptured part of the scale 

 from left-hand to right-hand margins. Sometimes they can be 

 followed even around the unsculptured part. Annuli are 

 clearly separated from each other and do not ordinarily 

 meet at any point. If an annulus has formed , it is present 

 in all the normal scales of an individual ." 



In the scales of fish older than six years, the peripheral annuli are 

 crowded and frequently irregular, so that they cannot be made out equally 

 well entirely around their course, particularly in very old specimens. Oi 

 such scales, the annuli are generally most distinct at the antei'ior "cor- 

 ners" of the soulptured part of the scale (see figure 1). There they are 

 most easily located and counted, and thence they can be traced, even though 

 with difficulty, entirely around the sculptured part of the scale. The 

 criteria of a false year ring, given previously (Walford and Mosher, 19h3) 

 apply for older fish as well as for yoimger ones. These are: 



"They are merely short or unassociated arcs; or if they com- 

 pletely circle the sculptured part, they are usually not 

 concentric with the margin; furthermore, they frequently 

 join an annulus at the base. In any case, they may be lesg 

 distinct than an annulus, being va2\ie and indefinite, or 

 they may be much more distinct, having a scar-like Appear- 

 ance; that is, v/ith very pronounced irregularities of pattern, 

 the folds being brbken or othein/ise obviously abnormal. 

 They rarely appear in all the scales of an individual ." 



ERROR OF INTERPRETING THE ANNULUS 



Having established certain definitions of an annulus and of an adventi- 

 tious, mark, it is pertinent to learn how accurately they can be employed. 

 Expected errors are of two typea: those caused by faults of the scale and 

 those caused by faults of tht3 reader. 



0f the first type are those cases in vitiich a scale fails to record a 

 year mark; also those in which an adventitious mark majr look enough like 

 an annulus easily to be mistaken for one. These cases of scale faults pro- 

 vide exceptions to our criteria of true and false annuli, and they are as 

 liable to the same interpretation in a second examination as are normal 

 scales. The percentage of errors arising from these causes probably can 

 never bo directly measured . 



100 



