MOOK, VARIATIOX IX SPIRIFER MUCRONATUS 



187 



attenuatus 



profundus thedfordense 



Primitive form, of Spirifer mucronaius 

 Fig. 6. — Diagram showing the relationships of the fiie mutations of Spirifer mucronaius 



The tendency in development has been to reduce the shell index, to 

 reduce the number of plications,- to lose the groove and plication on fold 

 and sinus, to strengthen the growth lines and individual plications, to 

 deepen the sinus, and to a certain extent to reduce the actual width of 

 the shell. 



