Age-Detepmination, Growth and Symmetry 3= 



their first, nineteen in their second, three in their third, and one in its 

 fourth year. If this material is representative the death rate is ex- 

 tremely high compared with that of C. macleayanum on the arctic coast 

 of Alaska. 



The average length at the first winter period of the disk of seven 

 individuals from Departure bay on the inner coast of Vancouver island 

 that were more than one year old, was, as determined by calculation 

 from the relative position of the first winter line or check, 9 millimetres, 

 the range being from 6.6 to 10.5 mm. Eleven individuals from Ucluelet 

 on the outer coast of Vancouver island had an average length for the 

 first winter period of 12.6 mm., the range being from 6.7 to 15. i mm. 

 This difference may be significant, as the inner coast of the island sup- 

 ports a colder water fauiia than does the outer coast. 



The few individuals of more than two years in age that have been 

 available, exhibit a marked decrease in growth after the second year. 

 We may instance the oldest one obtained, which had a disk 26 mm. long. 

 The increase in the disk in successive years was, as calculated, 10.2, 8.8 

 4.1 and 2.9 mm., of which the last increment may not have been com- 

 plete. The growth of C. productum may, therefore, be considered as 

 slight after the second year. 



It may be noted that also in this species the amount of growth 

 increases with the distance from the apertures, not only circularly with 

 the aperture as the centre, which is a necessity if the disk is to remain 

 flat when contracted, but also radially. In one individual, of which the 

 disk is shown in Plate I, fig. 3, a very peculiar growth was noted. At 

 least during the second year all of the extrasiphonal plates grew rela- 

 tively much more ajong their right than along their left borders. What 

 factor may have produced this we are unable to suggest. 



It may be noted that the youpg individuals less than one year old 

 had by July or August nearly completed their full growth for the year 

 as calculated from older specimens. This indicates that the season of 

 growth is the spring and early summer. 



Chelyosoma columbianum. Huntsman. 



The material at hand consists of a rather large number of individuals 

 from British Cblumbia with disks ranging from 10 to 18 mm. in length 

 which were collected during the summer seasons of the years 1908 and 

 1909. The body is almost as depressed as that of C. macleayanum, and 

 to some extent correlated with this condition there is a marked asym- 

 metry of the disk consisting of a pronounced development of the posterior 

 end and of the left side. As in C. macleayanum the disk is relatively 

 large. 



