253 
of the plate. It is at once evident that the umbones of the five 
primary plates are apical, i. e. that calcification is continued only 
along the lower sides of the plates. 
As soon as the calcareous deposits become evident outside of 
the margins of the primordial valves, the next plate, viz. rostrum, 
Fig. 14. Mitella polymerus f. typica, development of the skeleton. La Jolla. a pupa 
just attached ; b pupa with primordial valves; c pupa cover thrown off, rostrum de- 
veloped ; d specimen with upper latus only ; e somewhat aberrant specimen with 
three upper latera and subrostral latus ; f normal specimen with two of the lower 
latera developed ; g first peduncular scale appearing below rostrum, subcarina well 
developed. [All figures X 33]. 
makes its appearance (Fig. 14c); it is very soon followed by an 
upper latus, below the interval between tergum and scutum. A 
little later we can also distinguish a carinal, and a rostral latus; 
at this time the first plate of the lower row of latera moreover 
makes its appearance below the interval between rostrum and 
rostral latus. 
Generally the following latera of the lower row appear all but 
simultaneously with the last named lower latus, and now also the 
subcarina (Fig. 14 g) is observed as a rather obvious plate. 
Not until the subcarina is developed do the scales of the ped- 
uncle commence their development. First those of the ventral 
(rostral) side develop; little by little the lateral scales appear, the 
later the nearer the carino-sagittal line. Owing to the growth of 
