255 
the statement of previous authors that new scales normally arise 
farther down on the peduncle. 
The growth of the capitulum plates is due to apposition. New 
layers of carbonate of lime are deposited along the inner side of 
the plates, and particularly along the margins of the plates facing 
the transition from capitulum to peduncle. Umbo of the primary 
plate thus becomes apically seated. The lines of growth are ir- 
regular, and do not stand in any apparent connection to outer 
Fig. 16. Mitella polymerus f. typica ; La Jolla. a peduncle scale from the basal part, 
b from - the middle part of the peduncle, c mandible, d maxilla. 
[a-b X 44, c-d X 22]. 
circumstances; their numbers therefore also differ in different 
plates of the same individual, also in the primary plates, and do 
not afford any base for a judgement as to the age of the spec- 
imen. 
It is evident from the facts here stated that a great likeness 
is seen in the skeletonal development of Mitella and Scalpellum 
(comp. also Broch 1912, 1921); the main difference is seen in 
the appearance of the first scales of the peduncle. The first — i. e. 
the dorso-basal — pairs of peduncle scales in Scalpellum appear 
already at the same time as the superior latus. In Mitella, on the 
other hånd, the first peduncle scales are not developed till the sub- 
carinal row of latera have appeared, and the first peduncle scales, 
which then appear, are the ventro-basal ones. 
