r 



s'^ umber 



Percentage 



Number 



Percentage 



23 



6. 



1 



62 



16.6 



5 



1. 



3 



69 



18.5 



2 



. 



5 



60 



16.1 



1 





2 



45 



12.0 













26 



6.9 













21 



5.6 













15 



4.0 













4 



1.0 













2 



.5 













1 



.2 



MOOK, VARIATION IN SPIRIFER MUGRONATUS 177 



Adult Neanic 



Index 



2.01-2.25 23 



2.26-2.50 



2.51-2.75 



2.76-3.00 



3.01-3.25 



3.26-3.50 



3.51-3.75 



3.76-4.00 



4.01-4.25 



4.26-4.50 



Total 372 99.6 372 99.2 



The curve of the adult shell indices of alpenense is fairly smooth. The 

 slight roughness or irregularity may be attributed to imperfection of 

 some of the specimens and to the difficulty of deciding, in some cases, 

 the anterior end of the pedicle valve. The neanic curve is much more 

 irregular, a condition due, no doubt, to the extreme difficulty of finding 

 exactly corresponding growth stages in a great number of individuals. 



From the size, characters and number of plications, shell indices and 

 characters of the fold and sinus, alpenense is favorably equipped to be 

 an ancestor of the mutations profundus and the df or dense. 



Mutation II. — Spirifer mucronatus mut. muUiplicaius Gimbau ms. 



This mutation occurs in the limestone beds in the Michigan region 

 above the shales containing alpenense. Shales bearing alpenense some- 

 times alternate with these limestones. It is a large mutation with many 

 fairly strong fiat plications. There are about 17 plications on each side 

 of the fold and sinus. The fold and sinus are broad and fiat, the sinus 

 bearing a well-developed plication and the fold a median groove. The 

 growth lines are strong and the young stages are often clearly distin- 

 guishable. Twenty-nine individuals of inulfipUcatus were measured. 



Maximum width 50mm. 



Minimum width 26 



Maximum lengtli 27 



Minimum length 15 



Highest shell index (adult) 2.32 



Lowest shell index (adult ) 1 . 46 



Average shell index (adult) (about) . . 1 . SO 



Highest shell index (neanic stage) 3 . 50 



Lowest shell index (neanic stage) 1. 78 



Average shell index (neanic stage) (about) . . 2.65 



