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out corresponding enlargement ef the pedicle, leaves the latter restricted 
to a noteh (delthyrium) in the posterior margins of the valves, providing 
the peripheral growth is about equal on all anterior and lateral radii. 
If the shell growth is greater in the anterior direction, the shell becomes 
pointed, the pedicle (posterior) end remaining of about the original 
width. If the shell growth is mainly in the lateral directions, the shell 
becomes wide, with a long straight hinge, of which the pedicle opening 
forms a very small proportion. Whatever may be the later growth of the 
shell, all the earlier stages are preserved, except in cases where the beaks 
are injured or resorbed by the encroachment of the pedicle in adult and 
senile stages. The growth of the shell is entirely by additions at the mar- 
gins or on the inner surface. It follows that the protegulum may in ex- 
ceptionally well preserved material be seen intact at the beaks of the 
adult shell. It is often seen at the apices of young shells. 
Searching for the phylogenetic significance of the protegulum, Beecher 
(6) ascertained that certain of the earliest known brachiopods approxi- 
mate very closely in form to the protegulum, and he selected the genus 
Paterina (I[phidea) as the radicle of the class. It has since been shown 
that Paterina is not the most primitive known brachiopod. It is still 
true, however, that the most primitive brachiopods known are of the 
same general form and type as Paterina, in fact they approximate more 
closely, if anything, than that genus, to the form of the protegulum. It 
may be very safely concluded, therefore, from the geslogical evidence, 
that the primitive brachiopod was actually of the type indicated by the 
protegulum. 
Beecher says of Paterina: “In mature specimens, all lines of growth, 
from the nucleal shell to the margin, are unvaryingly parallel and con- 
centric, terminating abruptly at the cardinal line. In other words, no 
changes occur in the outlines or proportions of the shell during growth, 
through the nepionic and neanic stages up to and including the com- 
pleted ephebic condition. The resemblance of this form to the protegulum 
of other brachiopods is very marked and significant, as it represents a 
mature type having cnly the common embryonal features of other genera.” 
Among the Brachiopoda, as among the Pelecypoda there are a number 
of forms in which the condition of very close fixation or of burrowing has 
1 Walcott (62) seems to reserve this distinction for his genus Rustella. Pater- 
ina is by him made a subgenus of the genus Micromitra. These forms are all 
placed in the superfamily Rustellacea. 
