Age-Determination, Growth and Symmetry 37 



The number of extrasiphonal plates is commonly increased by 

 divisions giving plates that do not touch either siphonal area. This 

 affects the parts of the disk most distant from the apertures, chiefly the 

 middle of each side, where such additior^al plates are occasionally to 

 be found in the two preceding species. In C. columbianum this process 

 is continued posteriorly, chiefly on the left side. Such divisions take 

 place circularly with the apertures as centres and are therefore in series 

 with the grooves separating the siphonal from the extrasiphonal plates 

 and with that limiting the disk itself. Occasionally this process results 

 in a division of one of the central plates, thus somewhat definitely 

 separating the two extrasiphonal areas. 



We have been unable to find lines of growth on any of the plates in 

 any of the individuals examined. It is quite unlikely that large yellow 

 individuals obtained in June, 1909, can have been less than one year old. 

 We suggest that growth in this species is limited to the first year. We 

 did not obtain any very small individuals, but several w^hich had only a 

 faint yellowish tinge were obtained in AYigust, and these we consider 

 to be less than one year old, that is the product of the same year. They 

 were nearly as large as the strongly yellow individuals. We conclude, 

 therefore, that this species completes its growth during spring and early 

 summer, that the yellow layer is not fully formed until a much later 

 period, and that growth does not take place after the first year. 



Conclusions. 



Lines of growth shown by the plates of Chelyosoma may be used for 

 the determination of age and for the calculation of growth in view of the 

 facts, (i) that the outer yellow layer of the test is hard and rigid, (2) that 

 it increases in extent by additions at the margins of each plate, and 

 (3) that it continues to thicken at the turned-in margin after the growth 

 for the season has been completed, thus marking the winter outline of 

 the plate. 



C. macleayanitm in the Arctic waters of Alaska shows a well sustained 

 growth for more than four years. 



C. productum in the waters of southern British Columbia grows 

 vigorously in each of the first two years of its life and continues to grow, 

 but much less vigoroulsy, beyond the third year. 



C. columbianum in the waters of southern British Columbia grows 

 rapidly during the first year and apparently not thereafter. 



The comparative rate of growth of the parts of the disk of Chelyosoma 

 is related to the apertures, and obeys the law that growth both radially 

 and circularly increases in amount with the distance from the apertures. 



