346 G. E. Beecher — Development of the Brachiopoda. 



The valves of Paterina are subecjual, the pedicle valve being 

 a little more elevated than the brachial. They are semiellip- 

 tical in outline. In mature specimens, all lines of growth, 

 from the nucleal shell to the margin, are unvaryingly parallel 

 and concentric, 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 nealogic 

 stages up to and including the completed ephebolic condition. 

 The resemblance of this form to the protegulum of other 

 brachiopocls is very marked and significant, as it represents 

 a mature type having only the common embryonal features 

 of other genera. It is of further importance as representing, 

 in many species, an early condition of nepionic growth sub- 

 sequent to the protegulum. during which the proportions 

 and features of the shell undergo no modification except 

 increase in size. This is termed the paterina stage. It is well 

 shown in the brachial valve of Orbicidoidea minuta, Hall, 

 Plate XVII, figure 5. 



Modifications from acceleration. — The modifications in the 

 form of the protegulum are due to the influence of accelerated 

 growth, by which nepionic and sometimes nealogic features are 

 pushed forward, or appear earlier in the history of the indi- 

 vidual, so as to become impressed upon the early embryonic 

 shell. Only a brief review of these changes will be noted here, 

 as a fuller description properly belongs under the discussions 

 of the various genera and families. Naturally, the greatest 

 departure from the normal protegulum is exhibited in the 

 most variable and specialized valve, the pedicle valve. The 

 nearly equi valve genera, as Lingula and Glottidia, present 

 almost no modification. In the ventral valve of Linnarssonia 

 and Orbiculoidea (Plate XVII, fig. 7), the protegulum has a 

 hinge more or less arcuate. Discinisca shows a subcircular 

 pedicle protegulum with a pedicle notch, and the evidence of 

 any hinge in the brachial is very slight, Plate XVII, figures 8, 9. 

 The discinoid character appearing in the second and third 

 nepionic stage of the paleozoic Orbiculoidea (Plate XVII, fig. 6), 

 has become so accelerated in neozoic and recent Discinisca as to 

 produce a discinoid protegulum. 



The strophomenoid shells usually retain a normal protegulum 

 in the brachial valve, but from the acceleration of the discinoid 

 stage in the pedicle valve, the protegulum has an abbreviated 

 hinge and arcuate hinge line, Plate XVII, figures 13, 14, 15. 



dissimilar valves, arcuate ventral beak, and mesial depression, could be devel- 

 oped only by passing through, several well-marked phases. This in itself seems 

 sufficient for a separation were no other characters present. 



