325 



out corresponding enlargement of the pedicle, leaves the latter restricted 

 to a notch (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 

 (G) ascertained that certain of the earliest known brachiopods approxi- 

 mate very closely in form to the protegulum, and he selected the genus 

 Paterina (Iphidea) as the radicle of the class. It has since been shown 

 that Paterina is not the most primitive known brachiopod. 1 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 geological 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 only 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. 



